Essentials of Anatomy and Physiology

intravenous lines. Such infections are a significant
problem in hospitals, and all those involved in any
aspect of patient care should be aware of this.

EPIDEMIOLOGY

Epidemiologyis the study of the patterns and spread
of disease within a population. This term is related to
epidemic, which is an outbreak of disease, that is,
more than the usual number of cases in a given time
period. An endemicdisease is one that is present in a
population, with an expected or usual number of cases
in a given time. Influenza, for example, is endemic in
large cities during the winter, and public health per-
sonnel expect a certain number of cases. In some win-
ters, however, the number of cases of influenza
increases, often markedly, and this is an epidemic.
A pandemic is an epidemic that has spread
throughout several countries. The bubonic plague
pandemic of the 14th century affected nearly all of
Europe and killed one-fourth of the population. Just
after World War I, from 1918 to 1920, an especially
virulent strain of the influenza virus spread around the
world and caused at least 20 million deaths. More
recently, an epidemic of cholera began in Peru in
January 1991, but soon became a pandemic as cholera
spread to neighboring South American countries.
To understand the epidemiology of a disease, we
must know several things about the pathogen. These

include where it lives in a host, the kinds of hosts it can

infect, and whether it can survive outside of hosts.

PORTALS OF ENTRY AND EXIT

The portal of entryis the way the pathogen enters a

host (Fig. 22–1). Breaks in the skin, even very small

ones, are potential portals of entry, as are the natural

body openings. Pathogens may be inhaled, consumed

with food and water, or acquired during sexual activ-

ity. Most pathogens that enter the body by way of

these natural routes are destroyed by the white blood

cells found in and below the skin and mucous mem-

branes, but some may be able to establish themselves

and cause disease. You may wonder if the eye can be a

portal of entry for other than eye infections, or you

may already know that it is a portal for many respira-

tory viruses. People may pick up cold or flu viruses on

their hands and then rub their eyes; the viruses are

then washed by tears into the nasal cavities, their pre-

ferred site of infection.

Insects such as mosquitoes, fleas, and lice, and other

arthropods, such as ticks, are vectors of disease. They

spread pathogens when they bite to obtain a host’s

blood. Mosquitoes, for example, are vectors of malaria,

yellow fever, and encephalitis. Ticks are vectors of

Lyme disease and Rocky Mountain spotted fever.

As mentioned previously, it is important to keep in

mind that many hospital procedures may provide por-

tals of entry for pathogens. Any invasive procedure,

whether it involves the skin or the mucous mem-

An Introduction to Microbiology and Human Disease 503

Mouth

(water or

food)

Bites of

vectors

Skin

breaks

Urinary or

reproductive

tract

Urine Reproductive

tract

Feces

Skin

contact

Blood

Nose Eye Respiratory droplets

A B

Figure 22–1. (A) Portals of entry.
(B) Portals of exit.
QUESTION:Are there any portals of
entry that are unlikely portals of exit?