The Infectious Disease Process: Why You Get Sick-Or Not

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A bloody ruptured MRSA (Methicillin Resistant Staphylococcus aureus cyst.
Ruptured MRSA (Methicillin Resistant Staphylococcus aureus) cyst. Image:

You are exposed to thousands of pathogens (disease-causing organisms) every day. But, you don’t get sick every day.  Why not?  What makes one exposure to disease different from another?

First, exposure just means that you have, in some way, come into contact with a pathogen, an organism capable of causing disease. Pathogens include many different types of organisms. Bacteria, viruses, fungi, protozoa, helminths (parasitic worms), and prions are known to cause disease.

Secondly, infection is not the same as disease. An infection is any entry and multiplication of a pathogen into the body of animals and plants. (This lesson will focus on infections of humans.) Note that the presence of a pathogen on the outside of the body is not an infection; this is just a contamination. Disease results from an infection when adverse symptoms appear in the infected human.


To better understand the infectious disease process, you should be able to:

  • Explain the difference between disease and infection.
  • Describe the five components necessary for the infectious disease process and explain the importance of their roles in infection and disease.
  • Compare and contrast acquired and innate immunity.


acquired immunity— the resistance to a disease that results from previous exposure to an infectious agent, its antigens, or by transfer of antibodies.

carrier—a person who is infected with a pathogen, has no symptoms, but is capable of passing the pathogen to other people.

disease—a condition in which the body cannot function normally due to infection by a pathogenic agent, genetic condition, nutritional deficiency, or an illness of an affected body organ.

herd immunity—resistance to an infectious agent by an entire group or community; a large number of immune persons in a community reduces the likelihood that an infected person will come into contact with a susceptible person among the population.

host immunity—resistance to infection and disease by a host organism.

infection— a condition in which an infectious agent invades the body and begins to multiply.

innate immunity— inborn physical, chemical, cellular barriers humans have at birth.

portal of entry—the pathway by which an infectious agent can enter its host. For example, the influenza virus’s portal of entry is host’s respiratory tract.

reservoir—in epidemiology, the location in which an infectious agent normally lives; includes humans, animals, plants, or the environment.

transmission—any mechanism by which an infectious agent is spread to a host.

zoonosis—a disease transmitted by an animal.


Getting sick is not as easy as you think.  The infectious disease process has certain requirements, and if any of the required components are not present, the infection process is disrupted and you may not get sick.

The five basic components to the infectious disease process are:

1) an infectious agent

2) reservoirs

3) portals of entry and exit

4) transmission

5) host immunity


Head of a parasitic tapeworm Taenia solium.
The head of a parasitic tape worm Taenia solium, a pathogenic agent.
Content Provider: CDC/Dr. Mae Melvin. Image:

An infectious agent is any organism (or particle) capable of causing infection and disease. Obviously, without a pathogen (disease-causing organism) infection and disease would not occur.

Infectious agents include prions, viruses, bacteria, protozoa, fungi, and helminths (parasitic worms).


A reservoir is the location in which an infectious agent normally lives. Reservoirs include humans, animals, plants, or the environment. If an infectious agent does not have the specific reservoir required by its life cycle, it cannot survive, multiply and infect. Natural reservoirs help complete the life cycle of the pathogen.

There are many different types of reservoirs.

Many animals serve as reservoirs for infectious disease agents. For example, the pathogen that causes bubonic plague (Y. pestis) has natural reservoirs that include marmots, rats, prairie dogs, chipmunks, and squirrels. Lyme disease is transmitted through the bite of an infected deer tick.

   Another type of reservoir is a carrier. Carriers are people who have the infectious agent inside them, have no symptoms of the disease, but are still able to infect other people who go on to develop disease.

Picture of a common house cat.
Cats infect humans when they shed large numbers of toxoplasmosis eggs after eating infected mice. Image:

Many animals are reservoirs and carriers of infectious diseases. A zoonosis is a disease transmitted by an animal. As discussed, many disease agents have complex life cycles; multiple reservoirs and vectors may be required for a pathogen and are very specific. If a required reservoir or carrier is not present, the pathogen may not be able to survive.

Inanimate objects can also carry disease and some infectious agents are able to live in or on water, air, food, or soil.


Did You Know?

The most famous carrier, Typhoid Mary, had no symptoms of the typhoid disease, yet shed the typhii bacteria for her entire life. She infected at least 53 people with typhoid fever.

Typhoid Mary is an example of a chronic carrier.  Although not the cause of her carrier condition, incomplete antibiotic treatment increases the chances that you will become a chronic carrier. It is important to finish all antibiotic medication as prescribed by your doctor even if you are feeling better.


 Portals of Entry

A portal of entry is any body structure through which a pathogen enters. For humans, portals of entry include the respiratory tract, conjunctiva (membranes covering the eye), urinary and reproductive tracts, gastrointestinal tract, skin, and placenta (mother-to-child).

If portals of entry are blocked, the disease cannot be transmitted.  Woman outside wearing a mask.For example, healthcare workers wearing rubber gloves block the transmission of infectious agents from one environment to another; wearing masks helps prevent the spread of respiratory diseases. Some diseases have preferred portals of entry; tuberculosis prefers to enter the body through the respiratory tract. Many diseases are spread in multiple ways and have multiple portals of entry.

Knowing the disease portals of entry can help prevent infections. Proper hygiene habits, such as good hand washing, can go a long way to keeping people infection-free.


Disease transmission is any method by which infectious agents are transferred to a host. Again, many diseases can be transmitted more than one way. Direct, indirect, mechanical, vehicle transmission and vector-borne transmission are types of disease transmission methods; each method is considered in detail when epidemiologists work to control an outbreak.

When epidemiologists analyze outbreak cases, patterns often appear. These patterns can reflect the transmission method and provide valuable information for identifying the disease more quickly.  Effective prevention and control strategies can then be implemented and the outbreak stopped more quickly.

(For more information, read “Disease Transmission Methods” in the Support Curriculum.) 

Host Immunity

   Host immunity is the resistance to infection and disease. Humans have both innate and acquired immunity that provides some protection against infectious disease agents. Depending on the type of immunity and host health conditions, a person could come into contact with an infectious agent and not get the disease.

   Innate immunity is the inborn physical, chemical, cellular barriers humans have at birth. These are very effective barriers to infections. Examples include mucus linings which trap infectious particles, cilia along the respiratory tract, the acid environment of the stomach, and various types of immune cells which engulf pathogens.

  Woman getting an injection in the arm. Acquired immunity is the resistance to a disease that results from previous exposure to an infectious agent, its antigens, or by transfer of antibodies. When you get certain diseases your body forms antibodies against the pathogen; if you encounter the pathogen again, your body recognizes the pathogen and antibodies formed from the first encounter fight it off. The same process works if you are immunized against the disease; if you encounter the pathogen, your body fights it off.