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 Resident Flora Healthy people live in harmony with most of the microorganisms that establish themselves on or in (colonize) nonsterile parts of the body, such as the skin, nose, mouth, throat, large intestine... read more ). 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 Defenses Against Infection Natural barriers and the immune system defend the body against organisms that can cause infection. (See also Lines of Defense.) Natural barriers include the skin, mucous membranes, tears, earwax... read more and see Defenses Against Infection Defenses Against Infection Natural barriers and the immune system defend the body against organisms that can cause infection. (See also Lines of Defense.) Natural barriers include the skin, mucous membranes, tears, earwax... read more ).
Certain microorganisms have the potential to be used as biological weapons Biological Weapons Biological warfare is the use of microbiological agents as weapons. Such use is contrary to international law and has rarely taken place during formal warfare in modern history, despite the... read more . These microorganisms include those that cause anthrax Anthrax Anthrax is a potentially fatal infection with Bacillus anthracis, a gram-positive, rod-shaped bacteria (see figure How Bacteria Shape Up). Anthrax may affect the skin, the lungs, or, rarely... read more 
, brucellosis Brucellosis Brucellosis is an infection caused by several species of the gram-negative bacteria Brucella and characterized by fever and bodywide symptoms. Brucellosis is acquired mainly by having contact... read more , hemorrhagic fever (Ebola virus infection Ebola Virus and Marburg Virus Infections Marburg and Ebola virus infections cause bleeding and organ malfunction. These infections often result in death. Marburg and Ebola infections are spread through handling live or dead infected... read more and Marburg virus infection Ebola Virus and Marburg Virus Infections Marburg and Ebola virus infections cause bleeding and organ malfunction. These infections often result in death. Marburg and Ebola infections are spread through handling live or dead infected... read more ), plague Plague and Other Yersinia Infections Plague is a severe infection caused by the gram-negative bacteria Yersinia pestis and often involving the lymph nodes and/or lungs. The bacteria are spread mainly by the rat flea. Depending... read more 
, smallpox Smallpox Smallpox is a highly contagious, very deadly disease caused by the variola virus. The disease is now considered eliminated. There have been no cases of smallpox since 1977. People can acquire... read more 
, and tularemia Tularemia Tularemia is infection that is caused by the gram-negative bacteria Francisella tularensis, which is acquired when people have direct contact with infected wild animals, usually rabbits, or... read more 
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.
Effects of Infection
Infections may affect only part of the body (a local infection) or the whole body (a systemic infection). Abscesses and urinary bladder infections are examples of local infections. Severe systemic infections may have life-threatening effects, such as sepsis or septic shock Sepsis and Septic Shock Sepsis is a serious bodywide response to bacteremia or another infection plus malfunction or failure of an essential system in the body. Septic shock is life-threatening low blood pressure ... read more .
Symptoms of infection can include fever Fever in Adults Fever is an elevated body temperature. Temperature is considered elevated when it is higher than 100.4° F (38° C) as measured by an oral thermometer or higher than 100.8° F (38.2° C) as measured... read more , a racing pulse, faster breathing, anxiety, and confusion.
Most effects resolve when the infection is effectively treated.
