Essentials of Anatomy and Physiology

we may not always realize that. A child’s strep throat
will probably not progress to meningitis, and bacterial
pneumonia does not have the very high fatality rate
that it once did. But, we must keep in mind that antibi-
otics are not a “cure” for any disease. An infection,
especially a serious one, means that the immune sys-
tem has been overwhelmed by the pathogen. An
antibiotic diminishes the number of bacteria to a level
with which the immune system can cope. Ultimately,
however, the body’s own white blood cells must elim-
inate the very last of the bacteria.

VIRUSES (SEE TABLE 22–4)

Viruses are not cells; their structure is even simpler
than that of bacteria, which are the simplest cells. A
virus consists of either DNA or RNA surrounded by a
protein shell. The protein shell has a shape that is
characteristic for each virus (Fig. 22–3). There are no
enzymes, cytoplasm, cell membranes, or cell walls in
viruses, and they can reproduce only when inside the
living cells of a host. Therefore, all viruses are obligate
intracellular parasites, and they cause disease when
they reproduce inside cells. When a virus enters a host
cell, it uses the cell’s chromosomes, RNA, and enzymes
to make new viruses. Several hundred new viruses may
be produced from just one virus. The host cell rup-
tures and dies, releasing the new viruses, which then
enter other cells and reproduce.
The severity of a viral disease depends upon the
types of cells infected. If the virus affects skin cells, for
example, the disease is usually mild and self-limiting,
such as chickenpox. Small numbers of skin cells are

not crucial to our survival, and these cells can be

replaced by mitosis. If, however, the virus affects nerve

cells, the disease is more serious and may be fatal.

Rabies is such as disease. Neurons are much more vital

to us, and they cannot be replaced once they die.

Some viruses, such as those that cause German

measles (rubella) and chickenpox, are able to cross the

placenta, that is, pass from maternal circulation to

fetal circulation. Although the disease may be very

mild for the pregnant woman, the virus may severely

damage developing fetal organs and cause congenital

birth defects such as blindness, heart malformations,

and mental retardation. In the most serious cases, fetal

infection may result in miscarriage or stillbirth.

Some viruses cause an initial infection, become

dormant, then are reactivated, causing another infec-

tion months or years later. The herpes viruses that

cause cold sores “hide out” in nerves of the face fol-

lowing the initial skin lesion. At some later time when

the host’s resistance is lowered, the viruses emerge

from the nerves and cause another cold sore. The

chickenpox virus, which most of us acquire as chil-

dren, is a herpes virus that may become dormant in

nerves for years, and then be reactivated and cause

shingles when we are adults.

A few human viruses are known to be tumor

viruses, that is, they cause cells to develop abnormally

and form tumors. The Epstein-Barr virus, which

causes mononucleosis in North America, is associated

with Burkitt’s lymphoma in Africa and with nasopha-

ryngeal carcinoma in China. There are environmental

factors, as yet unknown, that contribute to the devel-

opment of these cancers in specific parts of the world.

An Introduction to Microbiology and Human Disease 511

Figure 22–3. Viruses: rep-
resentative shapes and relative
sizes.
QUESTION: The influenza
virus shows the two parts of a
virus. What are these two
parts?