Agents of Infection and Some Diseases

Like the natural world around you, your own body is part of the environment. You’re the host to countless tiny animal organisms and plantlike organisms, plus some that are neither animals nor plants, all living together in your body and carrying out their own basic life functions of respiration, metabolism, excretion, growth and reproduction.

Six categories of these organisms can become pathogens, or agents of infections disease: bacteria, viruses, ricketisiae, fungi, protozoa, and worms.

Bacteria
Most plentiful of the microorganisms endogenous to humans are the various kinds of bacteria (singular: bacterium). Each of these tiny organisms consists of just one cell, with a protective cell wall and all the structures it needs to carry out its life functions. The bacteria fall into the category of flora or plantlike organisms.

Endogenous Bacteria and Disease
Most kinds of endogenous bacteria are not harmful, and some, in fact, are vital to our existence. The bacterium Excherichia colli (often calle E. Coli), for example, lives in our intestines and is essential to the synthesis of the B vitamins, others help to kill off foreign infectious organisms. But our own endogenous bacteria can cause disease if for some reason they get out of hand. Skin bacteria sometimes cause aene; mouth bacteria sometimes help cause pyorrhea, a serous gum disease: intestinal bacteria may get into the urethra, particularly in a woman, and cause urinary tract infections. Different strains of E.colli, which we may be exposed to through travel, may cause diarrhea until our system adjust to their presence.