Some bacteria secrete toxic substances that damage human tissues, while others become parasites within cells. Some bacteria form colonies within the body and disrupt normal function. Most pathogenic fungi parasitise tissues at or near a body surface, such as the skin or mucous membranes. A few fungi can become distributed throughout the body. Protozoa can infest body fluids and cells,and include amoebae, flagellates, ciliates, and sporozoa.
Infection is a fact of life.
Most interactions between humans and infective organisms go unnoticed -the body's defence mechanisms are able to prevent the organism from proliferating in or on the body. Only occasionally is the outcome an infectious disease. An infectious organism must be able to adhere to, colonize, or invade the host and proliferate at least to some extent. As a result, animals have evolved elaborate defence mechanisms at their various interfaces with the environment. If an infectious agent manages to penetrate the defensive barriers and enter the body tissues, the body reacts with an inflammatory response and a variety of immune responses.