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All living creatures share two basic objectives in life: survival and reproduction. This doctrine applies equally to all members of the living world, including bacteria, viruses, fungi, and protozoa. To satisfy these goals, organisms must extract from the environment essential nutrients for growth and proliferation. For countless microscopic organisms, that environment includes the human body. Table 12.1 illustrates common pathogens that invade humans. Normally, the contact between humans and microorganisms is incidental and, in certain situations, may actually benefit both organisms. Under extraordinary circumstances, however, the invasion of the human body by microorganisms can produce harmful and potentially lethal consequences. The consequences of these invasions are collectively called infectious diseases.

common pathogens that invade humans

All scientific disciplines evolve with a distinct knowledge and use of vocabulary, and the study of infectious diseases is no exception. The most appropriate way to approach this subject is with a brief discussion of the terminology used to characterize interactions between humans and microbes.
Any organism capable of supporting the nutritional and physical growth requirements of another is called a host. Throughout this chapter, the term host most often refers to humans supporting the growth of microorganisms. Occasionally, infection and colonization are used interchange-ably. However, the term infection describes the presence and multiplication within a host of another living organism, with subsequent injury to the host, whereas colonization describes the act of establishing a presence, a step required in the multi-faceted process of infection.
One common misconception should be dispelled from the start: not all interactions between microorganisms and humans are detrimental. The internal and external exposed surfaces of the human body are normally and harmlessly inhabited by a multitude of bacteria, collectively referred to as the normal microflora. Although the colonizing bacteria acquire nutritional needs and shelter, the host is not adversely affected by the relationship. An interaction such as this is called commensalism, and the colonizing microorganisms are sometimes referred to as commensal flora. The term mutualism is applied to an interaction in which the microorganism and the host both derive benefits from the interaction. For example, certain inhabitants of the human intestinal tract extract nutrients from the host and secrete essential vitamin by-products of metabolism (e.g.,vitamin K) that are absorbed and used by the host. A parasitic relationship is one in which only the infecting organism benefits from the relationship and the host either gains nothing from the relationship or sustains injury from the interaction. If the host sustains injury or pathologic damage in response to a parasitic infection, the process is called an infectious disease.
The severity of an infectious disease can range from mild to life threatening. Severity depends on many variables, including the health of the host at the time of infection and the virulence (disease-producing potential) of the microorganism. A select group of microorganisms called pathogens are so virulent that they are rarely found in the absence of disease. Fortunately, there are few human pathogens in the microbial world. Most microorganisms are harmless saprophytes, free-living organisms obtaining their growth from dead or decaying organic material in the environment. All microorganisms, even saprophytes and members of the normal flora, can be opportunistic pathogens, capable of producing an infectious disease when the health and immunity of the host have been severely weakened by illness, malnutrition, or medical therapy.