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Infectious Diseases Overview

The terms “infection” and “disease” are not synonymous. An infection results when a pathogen invades and begins growing within a host. Disease results only if and when, as a consequence of the invasion and growth of a pathogen, tissue function is impaired.

An infectious disease is a disease that is caused by a microorganism, such as a bacterium, virus, or parasite, that is not normally found in the body and is capable of causing infection. Our bodies have defense mechanisms to prevent infection and, should those mechanisms fail, to prevent disease after infection occurs. Some, but not all, infectious diseases are contagious, meaning they can spread from person to person. Other infectious diseases can spread from animals or insects to humans, but not from person to person.

Infectious diseases are responsible for more than 25% of 57 million annual deaths worldwide. 2/3 of 9 million pediatric deaths yearly are from infectious disease. Some of those deaths could be prevented with vaccines. Some infectious agents are easily transmitted (that is, they are very contagious), but they are not very likely to cause disease (that is, they are not very virulent).


Microbes That Cause Infectious Diseases

There are five major types of infectious agents: bacteria, viruses, fungi, protozoa, and helminths. In addition, a new class of infectious agents, the prions, has recently been recognized.


Public Health Measures to Prevent Infectious Diseases

Developed countries have regulations that help protect the general public from infectious diseases. Public health measures typically involve eliminating the pathogen from its reservoir or from its route of transmission. Those measures include ensuring a safe water supply, effectively managing sewage treatment and disposal, and initiating food safety, animal control, and vaccination programs.



Vaccines can prevent outbreaks of disease and save lives. When a critical portion of a community is immunized against a contagious disease, most members of the community are protected against that disease because there is little opportunity for an outbreak. Even those who are not eligible for certain vaccines–such as infants, pregnant women, or immunocompromised individuals–get some protection because the spread of contagious disease is contained. This is known as “community immunity.”


Treatment of Infectious Diseases

While literally meaning “destroyer of life,” the term “antibiotic” has become the most commonly used word to refer to a chemical substance used to treat bacterial infections. The term “antimicrobial” has a somewhat broader connotation, generally referring to anything that inhibits the growth of microbes.


Sources: AIDSinfo Glossary,

Infectious Disease: A worldwide Problem, NIH Center for Infectious Disease Imaging

Community Immunity (“Herd” Immunity), National Institute of Allergy and Infectious Disease

Emerging and Re-emerging Infectious Diseases, National Institute of Allergy and Infectious Disease


Hope Through Research – You Can Be Part of the Answer!

Many research studies are underway to help us learn about infectious diseases. Would you like to find out more about being part of this exciting research? Please visit the following links:


For more information:

Go to the Infectious Diseases health topic.