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Bacteria Structure and Characteristics

Check for Understanding
1. Name and describe three bacterial shapes.

coccus—round-shaped bacterium

bacillus—rod-shaped bacterium

spirillum—spiral-shaped bacterium

 Describe 4 ways bacteria get energy.

  • photoautotrophs—bacteria that are photosynthetic. They use sunlight for energy and carbon dioxide as a carbon source.
  • chemoautotrophs—bacteria that synthesize their own organic compounds using only carbon dioxide as the carbon source and an inorganic substance as an energy source; they obtain energy from chemical reactions.
  • photoheterotrophs—bacteria that use the sun’s energy for photosynthesis and fatty acids, complex carbohydrates, and other compounds as carbon sources.
  • chemoheterotrophs—bacteria that are parasites or saprobes. Parasites live on or in a living host and saprobes get nutrients from wastes or remains of other organisms.
  1. What is a bacterial biofilm and what is its significance in infectious disease?

A slime layer or biofilm allows the bacteria to stick to materials and each other to form microcolonies within the slime layer. Bacterial biofilms are important considerations in some health conditions because they cause infections to be much more difficult to treat.

  1. Complete the following table:




● allows some bacteria to stick to surfaces such as teeth and mucous membranes

● helps some bacterial species avoid being engulfed by infection-fighting cells

● helps prevent the bacteria from drying out in some environments


biofilm allows the bacteria to stick to materials and each other to form microcolonies
cell wall


a semi-rigid permeable structure just inside the capsule that surrounds other bacterial structures and allows the bacteria to keep its shape.


a whip-like structure used for movement
plasma membrane


● provides support

● allows movement into and out of the cell; provides the selective permeability that allows metabolic function

● participates in energy-providing functions, synthesis of membrane lipids, and motility functions.




a gel-like substance inside the plasma membrane, it is the site of metabolic and reproductive processes
chromosome material


genetic material of the cell (DNA)


site for protein synthesis


extrachromosomal genetic material (DNA)

 5. Draw and label the structures of a typical bacterium.


Source: Boundless. “The Cell Wall of Bacteria.” ,

 The Body’s Defense Mechanism

Check for Understanding:

  1. Why is it good to have both non-specific and specific defense mechanisms in the body?

Non-specific defense mechanisms allow the body to protect itself against a wide range of potential pathogens and do not require previous exposure to the pathogen to create the defense (as in the production of antibodies in response to an antigen).

Specific defense mechanisms allow the body to protect itself against specific disease-causing pathogens and in some cases provide lasting immunity to the disease.

  1. Complete the table of non-specific defense mechanisms.
Defense Mechanism Function
Species resistance Provides resistance to a species against a disease that another species can contract
Mechanical barriers Physical barriers, such as the skin and mucus membranes that block the entrance to pathogens
Enzymatic action Protection against disease due to the presence of enzymes
Interferon Proteins produced by some cells in response to viruses
Inflammation Tissue response to injury and infections to disease-causing organisms
Phagocytosis The engulfing of pathogens by specialized cells in the blood
  1. Describe three different types of cells that function in specific defense immune responses.

lymphocyte—a type of white blood cell that functions to produce immunity.

monocyte—a type of white blood cell that can engulf larger foreign particles in the blood.

neutrophil—a type of white blood cells that can engulf smaller foreign particles in the blood.


  1. T-suppressor cells interact with B-cells to stop the formation of antibodies. When would we want this to happen?

Supressor T-cells stop the immune system from acting when there is not a threat to the body. An abnormal immune response can be harmful to the body. Autoimmune diseases occur when your immune system attacks your body by mistake.


Disease Transmission Methods

Check Your Understanding:

  1. Describe 4 different methods of disease transmission through contact.
  • Direct—requires physical contact between hosts.
  • Indirect—contact with body fluids or tissues of an infected individual.
  • Droplet—large infectious particles and sprayed into the air from the respiratory tract of an infected individual.
  • Droplet nuclei—small infective particles that are suspended in the air, taken in by a host, and are capable of travelling to the lungs.
  1. What is a vector and how does understanding the vectors in the life cycle of pathogenic organisms help control outbreaks?

A vector is any living organism that carries a disease-causing agent from one host to another in the life cycle of a pathogen.

A complete understanding of the vectors in the life cycle of pathogenic organisms helps control outbreaks. Vectors are specific for each pathogen. If the specific vector needed by the pathogen is absent, the pathogen cannot survive. The outbreak ends. Getting rid of the vector in the environment stops the disease.

  1. How does knowing the disease transmission method help epidemiologists control an outbreak?
    If an epidemiologist knows how the disease is transmitted, he/she can take steps to identify the source of the infection, stop further transmission, and control the outbreak.


  1. Describe three methods of indirect disease transmission.

  By vectors—a vector is a living organism such as an insect or arthropod that carries a disease-causing agent from one host to another in the life cycle of a pathogen.

  By vehicle-borne transmission—vehicle-borne transmission occurs when a non-living object that carries a disease-causing agent from one host to another in the life cycle of a pathogen.  

    By airborne transmission—pathogens are suspended in the air and enter a body through the respiratory tract.

  1. Give an example of control measures that would be used for each example below:
    a pathogen is transmitted by droplet infection from an infected respiratory tract.
  • behavior changes (sneezing or coughing in a manner that does not spread pathogens)
  • people could wear respiratory masks a pathogen is transmitted through the bite of an insect.
    • wear protective clothing
    • use insect repellant
    • use pesticides to kill insect vectors
    • drain swamps and marshes if vectors live in those areasa pathogen is transmitted through contaminated water.

Ensure proper sanitation of wastewater and prevent contamination of public water supplies.
a pathogen is spread by contact with an inanimate object.

Thoroughly clean objects that may be infected with a pathogen with a disinfectant that you know can kill the pathogen.
a pathogen is spread through contact with body fluids of an infected person

  • Avoid contact with infected blood or body fluids or objects that have been contaminated with blood or body fluids of an infected person.
  • Disinfect all surfaces that may have come into contact with contaminated materials. Disinfect means to use chemicals that can kill pathogens. The type of disinfectant depends on the pathogens present. 
  • Safely dispose of contaminated materials.
  1. The Black Death Case Study:
    Critical thinking: What does the case study illustrate about transmission? How does this case study illustrate that transmission by vectors is not always as straight-forward as it appears?

Possible answers:

The case study points out that transmission of a disease can occur through different means.  It was once thought that rats carried the disease, and while that is true, it was not known that fleas become infected with the plague-causing bacterium, Y. pestis, after feeding off the blood of an infected animal. The bacteria reproduce in large numbers in the flea and infect rats or humans when they bite the victim to feed.

The flea’s interaction with the bacteria is also not a straight-forward mechanism. Y. pestis bacterial cells clump together and attach to the proventriculus, a valve between the flea’s midgut and the esophagus. This blocks the flea’s meals from traveling through the digestive system and the flea feeds more often in an attempt to get food. More bacteria enter the bite wound when the flea feeds and the victim is infected with plague.  The presence of this mechanism in the flea produces a more effective transmission method and higher rate of successful infections.

Critical thinking: Consider the life cycle and transmission methods for the spread of the Black Death. What prevention and control measures would you recommend based on the epidemiology of Y. pestis?

Control the rodent population: eliminate places where rodents can live and find shelter near your home or work place, remove brush and rocks, clear firewood, reduce possible food sources for rodents, and patch any areas of your home that may allow rodents to enter.

Use insect repellent to control the flea population: Use repellents if you are hiking or camping in an area known to have plague, apply repellent to skin and clothing, keep fleas off your pets, do not let pets roam in open areas known to have had plague.

Pathogenic Fungi

Check Your Understanding

  1. Differentiate members of the Kingdom Fungi from the members of each of the other kingdoms.

fungi-some are eukaryotic, some prokaryotic; some have a cell wall; do not have chlorophyll; cannot undergo photosynthesis; some unicellular, some multicellular; have sexual and asexual reproductive phases

bacteria-all prokaryotic

protists-some prokaryotic, some eukaryotic; some unicellular, some multicellular

plants-undergo photosynthesis to make their own food; have chlorophyll, have a cell wall

animals-do not make their own food; are multi-cellular; are motile; all sexual reproduction

  1. How are the fungi classified? List the groups according to this classification and give an example of each.

Fungi are classified by how they reproduce sexually.

Four groups of fungi based on sexual reproductive structures are:

1) sac fungi—produce spores by asci—examples are Coccidioides sp. and Aspergillus sp.

2) club fungi—produce spores by basidium—example is Cryptococcus neoformans

3) sporangia fungi—produce spores by sporangia—example is Rhizopus sp.

4) Imperfect fungi—sexual stage of reproduction has not been observed—example is Penicillium sp.

  1. People are constantly exposed to fungi and their spores, yet fungal diseases are relatively rare in the general population. Why?

Most people’s immune systems respond to fight off most fungal infections and disease does not occur.

  1. Under what circumstances will an aerial borne fungus cause disease in humans?

As people are constantly exposed to airborne fungi, their immune system responds immediately, rejecting the invader. When the immune system is compromised in some way, infection may be established. Causes of immune suppression include disease (HIV, cancer, diabetes), treatments (cyclosporine A, prolonged use of antibiotics,) or biotic factors (pregnancy, or very old or young age.)


  1. Critical thinking: Fungi tend to reproduce sexually when nutrients are limited or other conditions are unfavorable, but reproduce asexually when conditions are more ideal. Why this is a successful strategy for the fungi?

Sexual reproduction increases genetic variation. This genetic variation may produce some progeny that are more suited to new or challenging conditions, such as limited nutrients. When the genotype is successful in a particular environment or an environment which is not challenging, creating new genetic combinations is not necessary and is a waste of favorable organism energy.



Check Your Understanding:

  1. Describe the body plan and structure of a flatworm.

Flatworms are bilateral with a head region. They have simple organs, a pharynx, and a gut. Most are hermaphroditic; individuals have both male and female sex organs. Proglottids are segments of the tapeworm body.

  1. How does the digestive tract of flatworms differ from the digestive tract of round worms? What advantage, if any, does this give roundworms?

Roundworms have a complete digestive system from mouth to anus. Flatworms have only one opening that serves as both a mouth and an anus.

Having a complete digestive tract provides more complete digestion of food with less nutrient waste.


  1. Name 3 types of parasitic flukes and describe the diseases they cause. What is the definitive host in all three types of flukes?

Liver fluke infections cause fever, an enlarged liver, abdominal pain, anorexia, nausea, and vomiting.

Blood fluke infections cause rash, fever, chills, cough, and muscle aches. Chronic infections cause abdominal pain, bloody stool and urine, increased risk of bladder cancer, and seizures and paralysis, if eggs travel to the brain.

Lung fluke infections cause chronic coughing, bloody sputum, chest pain, fever, collapsed lungs, and seizures and vision impairment if worms travel to the brain.

Humans are the definitive host in all three types of fluke infections.


  1. Describe ways of preventing and controlling parasitic worm infections based on how they are transmitted and the characteristics of their life cycles.
  • Correct poor sanitation procedures to prevent spread of some types of parasitic worm eggs.
  • Make sure all meat is cooked thoroughly before eaten to prevent infection of pork and beef tapeworms.
  • Use proper personal hygiene, such as washing hands after defecating or not touching objects that may be infected with parasitic worm eggs which could be transferred from the hands to the mouth.
  • Take extra precautions to avoid infection if a family member has a parasitic worm infection.
  • Don’t walk barefoot outside in areas where hookworms may be present.
  • Always wash hands thoroughly before preparing food.
  • Get rid of parasitic worms in the environment by disrupting their life cycles, if possible. For example, getting rid of black flies, the intermediate host of the worm that causes river blindness, will help reduce the number of cases of the disease.

Critical thinking:

  1. Many groups of parasitic worms are hermaphroditic. How is being hermaphroditic an advantage for an organism?

During times of limited availability of mating partners, hermaphroditism is an advantage. If there is low population numbers or if the species has limited mobility, 1) hermaphrodites can adjust resources between males and female functions, 2) the likelihood of meeting a partner is higher because all individuals are prospective mates, and 3) many hermaphroditic animals can self-fertilize eliminating the need to find any suitable partner. 

Mate search has high costs in time and energy. This is offset by requirements for production of gametes.  In species with low mate search efficiency, hermaphroditism becomes a more stable mechanism for reproduction. In species with high mate search efficiency, hermaphroditism is a less stable mechanism in favor of separate sexes.

 Creating and Interpreting an Epicurve 

Common source with continuing exposure outbreaks:

When would you say exposure ends?  Around Jan 23. The incubation period is around 2-3 days and no new cases occur after the last incubation period. Cases start to drop.

Intermittent outbreaks:

Why would this not be a person-to-person transmission? 

This does not seem to be person-to-person transmission because there would not be gaps in between cases. With person-to-person transmission, cases would continue to increase as the condition spread through and beyond the community.


  1. What is the type of source of disease spread in the epi-curve below? Explain.

Answer: point source exposure

  1. What is the type of source of disease spread in the epi-curve below? Explain.

Answer: Common source with intermittent exposure


How Microorganisms Cause Disease

Check Your Understanding

  1. Describe transmission methods that allow pathogenic microorganisms to enter the body.

Direct transmission methods include direct contact with soil, plants, or people infected with a pathogen.

Indirect transmission methods are methods by which a pathogen travels to a host. These methods include those that are:

  • airborne—an infectious agent is carried from a source to a host while suspended in air particles.
  • vector borne—an infectious agent is carried by a live carrier, such as a mosquito, flea, rat, or tick.
  • vehicle-borne—an agent is carried by an inanimate object, such as food, water, blood or items such as surgical instruments.
  1. How do microbes cause disease?

In general, microbes cause disease by entering a host, causing an infection (the pathogens multiply), and disrupting cellular processes.

  1. What are exotoxins and endotoxins and how do they cause disease?

Exotoxins are poisonous substances made by pathogenic bacteria released into cells that disrupt cellular function. Examples of bacteria that produce exotoxins include Clostridium botulinum that produces the deadly botulism toxin, Corynebacterium diphtheria which produces diphtheria, and Clostridium tetani which produces tetanus.

Endotoxins are poisons that are part of a pathogenic bacterial cell well. Unlike endotoxins, endotoxins are not released from the bacteria until the bacteria adhere to the surface of a body cell.

  1. Describe how knowing the transmission method of specific pathogens could allow you to avoid disease.

For example, if a pathogen is airborne, you can take precautions to avoid being in environments in which people have an airborne disease. If a pathogen is vehicle-borne, you can disinfect the surface, thoroughly cook your food, make sure your water supply is safe, or avoid contact with blood and body fluids. Knowing how the pathogen is transmitted allows you to take appropriate precautions.


  1. Why are diseases produced by pathogens so different?

Pathogens are genetically different. Their genetic make-up dictates their life cycles, including their hosts and the body cells they infect. Therefore, some pathogens infect lung cells and cause pneumonia, some infect mucosal cells of the urinary tract and cause infections, and some infect the gastrointestinal tract and cause nausea, vomiting and diarrhea. Some pathogens can infect more than one type of tissue.


How Vaccines Prevent Disease

Check Your Understanding

1) Explain how your blood cells work to fight off disease.

White blood cells are the body’s disease-fighting cells. Some leukocytes circulate through the body, encounter diseased, damaged, or dead cells and destroy them. Other cells, lymphocytes, provide protection against specific diseases. 

The most important types of lymphocytes are B cells and T cells.

  • B cells—make antibodies after being exposed to specific antigens. Vaccines stimulate B cells to produce antibodies and a “memory response” that allows the body to recognize a disease-producing organism the next time it comes into contact with it.
  • T cells—destroy infected or abnormal cells by releasing toxic chemicals.


2) How does a vaccine produce immunity to a disease?

Vaccines trigger the immune system’s disease fighting cells to produce antibodies. They take advantage of the body’s natural abilities to fight disease. Just as the body produces antibodies that react to invading microorganisms, vaccines are designed to boost the immune system’s ability to quickly recognize microbes and destroy them before the disease process begins.

3) How would passive immunity help a person with a compromised immune system?

If a person with a compromised immune system was unable to produce their own antibodies in response to a vaccine (vaccine-induced immunity) or to a pathogen (natural immunity), they could still receive the antibodies from another source to help fight the disease (passive immunity).

4) Name the type of immunity that is produced by each of the following:

1) a person has ebola

active, natural immunity

2) a person receives an immunization against whooping cough

active, vaccine-induced

3) a baby has antibodies that he got from his mother through the placenta before birth


4) a person with botulism receives immunoglobulin therapy


5) a person receives antibodies from a person who has survived a disease


6) a baby has antibodies that she got through mother’s milk


7) a person receives immune serum globulin after exposure to hepatitis


8) a person gets a booster shot of tetanus antitoxin

active, vaccine-induced



Check for Understanding:

  1. Describe one way of classifying the protozoans and list the groups of protozoans according to that classification.

Protozoans are classified by their movement.  

zooflagellates—move by flagella
sarcodines—move by extensions of the cytoplasm
ciliates—move by cilia
sporozoans—no movement


  1. Briefly describe examples of pathogens and the diseases they cause for each group of pathogenic protozoans.

Giardia lamblia—causes severe diarrheal disease
Leishmania sp.—three types cause diseases of specific body parts:
            cutaneous—affects the skin
visceral-after internal organs, such as the spleen, liver, and bone marrow
mucosal—affects the nose, mouth and throat

   Trichomonas sp.—causes a sexually transmitted disease
symptoms include burning after urination, itching of urethra, and discharges from the urethra in men and vaginal itching, foamy greenish-yellow vaginal discharge, and vulvar itching in women.

Entamoeba histolytica—amoebic dysentery
causes painful cramping, diarrhea, bloody stools and urine, anemia, fever, chills, weight loss, swelling of the liver, and jaundice. Rarely, cysts travel to the lungs, brain, and other body organs.

Balantidium coli—diarrhea, abdominal pain

    Plasmodium sp.—malaria
causes fever, chills, convulsions, headaches, muscle pain, nausea, sweating, vomiting, coma, and bloody stools. Symptoms occur in cycles of 48-72 hours.

    Toxoplasmosis—headache, fever, muscle pain and sore throats


  1. Consider how parasitic protozoans are transmitted and describe how protozoan diseases may be prevented or controlled.
  • Make sure water supplies are not contaminated with protozoans
  • Do not eat raw or undercooked contaminated food
  • Clean all surfaces that may have come into contact with pathogens
  • Get rid of insects in the environment that may spread disease organisms (such as sand flies and Anopheles mosquitos)
  • Practice good hygiene, such as washing hands after defecating, before eating, and before food preparation.


The Epidemiological Process

Check Your Understanding:

  1. What is the importance of following the epidemiological method when investigating possible disease outbreaks?

Epidemiology uses the scientific method for disease investigation. It is important because it allows the rapid and accurate identification of the pathogen causing the disease, its origin, and its transmission mechanism. Accurate identification of issues involved in disease occurrence and spread allows the outbreak to be controlled quickly and allows patients to be treated quickly and correctly.

  1. A sudden rash of similar symptoms is reported to a county health department by several hospital emergency rooms, doctors’ offices, and healthcare facilities.
  2. Write a case definition for the following illness:

Representative symptoms of 28 cases of disease received by a county health department in Ohio:

  • Severe diarrhea, vomiting, abdominal pain for last 3 days
  • Bloody diarrhea, vomiting, abdominal cramping, low-grade fever for 4 days
  • Diarrhea, vomiting, abdominal pain, dark urine, low urine output for last 2 days
  • Severe diarrhea, blood in stools, abdominal pain, bloody urine, low-grade fever for last 3 days; symptoms getting worse
  • Diarrhea, vomiting, abdominal cramping for 5 days; symptoms are improving

Five people have been hospitalized due to dehydration.

Preliminary case definition:

Clinical illness is characterized by diarrhea (often bloody) and abdominal cramping and pain. Bloody stools, dark-colored urine, and low-grade fever may also be present. Illness lasts for at least 5 days.

Critical thinking: What are possible hypotheses for these symptoms?

Gastrointestinal flu, food poisoning

How would you test these hypotheses?

For food poisoning: Investigate if the patients ate at the same restaurants, bought the same food at grocery stores, attended the same events where food was prepared and offered, or went to the same venues, such as parks or theaters were food was offered.

For gastrointestinal flu or illness:  Investigate whether or not other family members “caught” the illness after the first family member got sick, find out if a similar illness was reported in the same schools or work places as the patients.

For both:  Order and analyze test results that may confirm either diagnosis. For example, tests on stool samples may show food poisoning bacterial contamination with Staphylococcus sp.  Gastrointestinal flu sample would not show bacterial contamination.

You find out the illnesses were food poisoning by E. coli and that the source was cheese bought at a large grocery store. The cheese was supplied by a major distributor which supplies cheese to multiple states in the Mid-west region.

Write a public health report which will alert the public to the E. coli outbreak. Your public health report should include details about symptoms of the disease, cause, how to avoid the disease, and what to do if you get sick.

Answers will vary.


The Immune Response

Check Your Understanding:

  1. List the function of each of the following cells:
  2. B cells-a type of lymphocyte that produces antibodies.
  3. b. helper T cells-lymphocytes that stimulate mitotic divisions that result in the production of increased numbers of T cells after contact with an antigen.
  4. effector cells-lymphocytes that actively engage a pathogen or foreign particle to engulf and destroy it.
  5. memory cells-lymphocytes that do not actively engage a foreign particle, but develop antibodies that are capable of recognizing the foreign particle when it is encountered in the future.
  6. e. cytotoxic T cells-T lymphocytes that produce chemicals that kill infected cells or tumor cells.
  7. What is immunological specificity?

The characteristic of the immune system by which white cells in the blood are able to target a particular type of pathogen when it appears in the body.

  1. What is immunological memory?

The characteristic of the immune system by which white cells in the blood are able to form antibodies against a particular disease which remain in the blood to fight that disease in the future.

  1. List steps in the immunological process upon introduction of a pathogen.

bacteria or viruses enter the body, lymph vessels carry infected interstitial fluid to lymph glands, macrophages in lymph glands engulf the foreign cells and enzymes break antigen molecules into pieces, antigen fragments bind to MHC molecules on the plasm membrane of nucleated body cells, antigen-MHC complexes are formed, lymphocytes recognize that these complexes are foreign and activate three types of cells:

   helper T cells— stimulate mitotic divisions and produce T lymphocytes
cytotoxic T cells—
produce chemicals that kill infected cells or tumor cells
B lymphocytes— produce antibodies that bind to the antigen

As the antigens are destroyed, fewer T and B cells are produced and the immune response stops. Chemical signals from cells suppress the immune response.

  1. List four places were pathogens could be found in the body.

blood, lymph, body tissues, inside cells


  1. Why don’t lymphocytes attack body cells?

Lymphocytes recognize body cells as “self” because of the presence of self-markers on lymphocytes.

Thinking critically

  1. If the lymph vessels pick up pathogen-containing fluid from an injury site or an infected area, and then carry the pathogen around the body via the blood and lymph fluid, why doesn’t this spread the pathogen throughout the body and result in infection and disease?

Lymph vessels carry fluid from an infected area to lymph glands or lymph nodes. Lymph nodes are packed with cells that initiate disease-fighting processes so the pathogens are not spread readily throughout the body.


  1. Under what situation would the recognition of a “non-self” cell not be advantageous?

An immune response—recognition of a non-self particle—is not advantageous to a person who has received an organ transplant. Cytotoxic T cells would contribute to the rejection of tissues and organs that are not recognized as “self” cells. The immune response would respond to the transplanted tissues as they would respond to an antigen.

Immune responses to non-matching blood transfusions can result in death of the recipient. While blood groups (A, B, AB, and O) are fairly easy to match, there are many other proteins on red blood cells that occur less frequently in the population.  These proteins are still recognized as antigens in non-matching transfusions and an immune response with killing and agglutination of blood cells will occur. In some cases, this makes finding a donor for critically needed blood difficult.

The Lymphatic System

Check Your Understanding:

  1. How does the lymphatic system work to defend the body against disease-causing agents?

Lymph carries proteins, water, and foreign particles such as viruses and bacteria. The structure of the lymph vessels allows these proteins and foreign substances to enter the capillary and eventually be carried to lymph nodes where white blood cells, such as lymphocytes and macrophages can destroy pathogens and damaged cells that may be present.

Foreign substances are filtered through the lymph tissues.

  1. Explain the disease-fighting function of the spleen.

Blood can be stored in the spleen, and in time of increased need, it can be released into circulation. Tissues in the spleen (white pulp) contain large numbers of lymphocytes while other tissues (red pulp) contain red blood cells, along with some lymphocytes and macrophages. Macrophages get rid of red blood cell fragments that have ruptured during passage through the capillaries. Macrophages also engulf and destroy bacteria.

  1. Explain the disease-fighting function of the thymus.

The thymus contains lymphocytes that originally developed in bone marrow and further develop in the thymus into T-lymphocytes which leave the thymus and provide immunity.

  1. Track the route of lymph through the body starting from the collection of lymph in the lymphatic capillaries in body tissues.

lymphatic capillaries, afferent lymphatic vessels, lymph nodes, efferent lymphatic vessels, lymphatic trunks, thoracic duct or right lymphatic duct, venous blood system, right atrium.


Types of Pathogens

Check Your Understanding:

List and describe the general structure of five types of human pathogens.

Viruses—made up of a protein coat (capsid) enclosing either DNA or RNA (the genetic code). Some viruses also have a lipid membrane. The protein coat has spikes and tails to help the virus bind to receptor sites on host body cells.

Prions—consist only of proteins in an abnormal, multiple folding pattern.  Prions are found most often in the brain and the abnormal folding leads to brain damage, neuron loss, and neurodegenerative disorders.

Bacteria—single-celled, prokaryotic organisms with distinct shapes. While there is variety within each group, bacteria are generally round (cocci), rod-shaped (bacillus), or spiral-shaped (spirilla). Bacteria have capsules surrounding an outer layer and can have flagella or cilia.

Helminths—invertebrates with worm-like, flat or rounded bodies. There is much diversity in the helminths group with very different characteristics, different hosts, and different life cycles. Helminths have egg, larva, and adult stages.

Fungi—single- or multi-celled, eukaryotic organisms that digest their food externally by secreting digestive enzymes and absorbing nutrients directly into their cells. They are non-motile and do not undergo photosynthesis. Fungi are classified by their reproductive structures as sac fungi, club fungi, sporangia fungi, and the “imperfect” fungi.


Viruses and Prions

Check Your Understanding

  1. Describe the structure of a virus.

Viruses are made up of a protein coat, or capsid, which encloses either DNA or RNA, the genetic code for the virus. Some viruses also have a membrane (made up of lipids) that covers the capsid, but not all viruses have this membrane.

Other features that may be part of the protein coat or lipid membrane include protein subunits in various shapes, spikes, and tails. The features are not additional components; they are part of the protein coat or viral genetic material. For example, the protein coat consists of one or more types of protein subunits arranged in various shapes, depending on the virus type. Some viral coats have spikes and tails. Spikes and tails help the virus bind to receptor sites on body cells.

  1. How does a virus cause disease in the human body?

Once inside the body cell, viral DNA or RNA can be replicated and thousands of new viral particles are produced. The host cell dies and the new viral particles infect more host cells. As more and more host cells are infected, more symptoms occur, and metabolism is disrupted.


  1. How does a virus multiply?

Five basic steps to viral replication are:

1) Attachment:  The virus attaches to a host cell. Any cell will do as long as the virus can chemically attach to specific molecular groups on the cell surface.

2) Penetration: The entire virus or just its genetic material penetrates the cell and enters the cell’s cytoplasm.

3) Replication and Synthesis: The viral DNA or RNA uses the host cell to produce thousands of copies of viral nucleic acids and proteins.

4) Assembly: The new viral nucleic acids and proteins form new viral particles.

5) Release: New viral particles are released from the cell, the cell dies, and the new particles go on to infect other healthy host cells.


  1. Describe the structure of a prion.

Prions consist only of protein, yet are the cause of fatal diseases. A prion protein has multiple folding patterns and is capable of causing abnormal folding of normal proteins found mostly in the brain.

  1. With respect to genetic composition, how are prions unlike any other type of infectious agent?

Prions are made up of scraps of proteins; viruses have a genetic structure (DNA and RNA) and a protein coat. Bacteria are prokaryotic cells and all other types of infectious organisms are eukaryotic.