Essentials of Anatomy and Physiology

many times, such people probably will not develop
clinical measles.
In contrast, the rhinoviruses that cause the com-
mon cold are not considered virulent pathogens, and
healthy people may have them in their upper respira-
tory tracts without developing illness. However,
fatigue, malnutrition, and other physiological stresses
may lower a person’s resistance and increase the like-
lihood of developing a cold upon exposure to these
viruses.
Once infected, a person may have a clinical
(apparentor symptomatic) infection, in which symp-
toms appear. Symptomsare observable or measurable
changes that indicate illness. For some diseases, sub-
clinical(inapparentor asymptomatic) infections are
possible, in which the person shows no symptoms.
Women with the sexually transmitted disease gonor-
rhea, for example, may have subclinical infections, that

is, no symptoms at all. It is important to remember

that such people are still reservoirs(sources) of the

pathogen for others, who may then develop clinical

infections.

COURSE OF AN INFECTIOUS DISEASE

When a pathogen establishes itself in a host, there is a

period of time before symptoms appear. This is called

the incubation period. Most infectious diseases have

rather specific incubation periods (Table 22–2). Some

diseases, however, have more variable incubation peri-

ods (see hepatitis in Table 22–2), which may make it

difficult to predict the onset of illness after exposure

or to trace outbreaks of a disease.

A short time called the prodromal period may

follow the incubation period. During this time, vague,

non-specific symptoms may begin. These include

An Introduction to Microbiology and Human Disease 501

Table 22–1 DISTRIBUTION OF NORMAL FLORA IN THE HUMAN BODY

Body Site Description of Flora

Skin

Nasal cavities

Trachea, bronchi,

and lungs

Oral cavity

Esophagus

Stomach

Small intestine

Large intestine

Urinary bladder

Vagina

Tissue fluid

Blood

Exposed to the environment; therefore has a large bacterial population and small numbers of

fungi, especially where the skin is often moist. Flora are kept in check by the continual loss

of dead cells from the stratum corneum.

Bacteria, mold spores, and viruses constantly enter with inhaled air; the ciliated epithelium limits

the microbial population by continuously sweeping mucus and trapped pathogens to the

pharynx, where they are swallowed.

The cilia of the trachea and large bronchial tubes sweep mucus and microbes upward toward the

pharynx, where they are swallowed. Very few pathogens reach the lungs, and most of these

are destroyed by alveolar macrophages.

Large bacterial population and small numbers of yeasts and protozoa. Kept in check by lysozyme,

the enzyme in saliva that inhibits bacterial reproduction. The resident flora help prevent the

growth of pathogens.

Contains the microorganisms swallowed with saliva or food.

The hydrochloric acid in gastric juice kills most bacteria. This may not be effective if large num-

bers of a pathogen or bacterial toxins are present in contaminated food.

The ileum, adjacent to the colon, has the largest bacterial population. The duodenum, adjacent

to the stomach, has the smallest.

Contains an enormous population of bacteria, which inhibits the growth of pathogens and pro-

duces vitamins. The vitamins are absorbed as the colon absorbs water. Vitamin K is obtained in

amounts usually sufficient to meet a person’s daily need. Smaller amounts of folic acid,

riboflavin, and other vitamins are also obtained from colon flora.

Is virtually free of bacteria, as is the upper urethra. The lower urethra, especially in women, has a

flora similar to that of the skin.

A large bacterial population creates an acidic pH that inhibits the growth of pathogens.

Small numbers of bacteria and viruses penetrate mucous membranes or get through breaks in the

skin. Most are destroyed in lymph nodules or lymph nodes or by wandering macrophages in

tissue fluid.

Should be free of microorganisms.