INFECTIOUS DISEASES
Terminology
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.
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.
