PATHOGENIC ORGANISMS
Pathogenic organisms are life forms that cause human disease. They range in size and complexity, and include molecules like proteinaceous particles (prions); viruses that are visible under an electron microscope; bacteria, fungi, and protozoan parasites that are sometimes visible to the naked eye; and multicellular parasites like tapeworms that may be many meters long. Many live in natural ecosystems, while others are commensal or parasitic on animals and/or humans. Only a few cannot survive independently of human hosts.
Pathogenic organisms can harm human health in several ways, including consuming nutriment intended for their host (tapeworms); producing poisonous metabolic products (staphylococcus, diphtheria, botulism toxin, and many others); destroying vital organs and tissues (prions, polio, rabies viruses); or interfering with body chemistry (toxic fungus). A few cause cancer (e.g. campylobacter). Their capacity to harm varies widely: The rabies virus virtually always kills its human victims, but seems to live harmlessly in many species of bats. Some obscure microorganisms that do no harm to healthy people can cause debilitating and ultimately fatal infections in persons whose immune defenses have been disrupted by the AIDS (acquired immunodeficiency syndrome) virus.
Rapid advances in microbiology, immunology, pharmacology, and other relevant sciences throughout the twentieth century have led to increasingly effective control measures against many pathogens. However, in biomass alone, microorganisms outweigh higher life forms, including humans, by several orders of magnitude, and they may number many billions of species. Eradication, or even elimination, of most pathogens from human ecosystems is thus not feasible. Humans must therefore learn to live in harmony with them as best they can. The best strategies generally are enhanced immunity and, where possible, avoidance of exposure to them and their harmful effects.
