Overview of infectious disease

[Becky]

Microorganisms are tiny living creatures, such as bacteria and viruses. Microorganisms are present everywhere. Despite their overwhelming abundance, relatively few of the thousands of species of microorganisms invade, multiply, and cause disease in people.
Many microorganisms live on the skin and in the mouth, upper airways, intestines, and genitals (particularly the vagina) without causing disease (see Resident Flora). Whether a microorganism lives as a harmless companion to a person or invades and causes disease depends on the nature of the microorganism and on the state of the person’s natural defenses (see Defenses Against Infection and see Defenses Against Infection).
Certain microorganisms have the potential to be used as biological weapons. These microorganisms include those that cause anthrax, brucellosis, hemorrhagic fever (Ebola virus infection and Marburg virus infection), plague, smallpox, and tularemia and those that produce botulinum toxin. Each of the diseases is potentially fatal and, except for anthrax, botulism, and tularemia, can be passed from person to person. Direct person-to person transmission of brucellosis is extremely rare.