Several of the human papillomaviruses have also been
associated with cancers of the mouth or larynx, and
three of these viruses are found in 90% of cervical car-
cinomas in women.
Important viral diseases are described in Table 22–4
at the end of this chapter.
Antiviral Medications
The treatment of viral diseases with chemicals poses
some formidable challenges. First, viruses are active
(reproducing) only within cells, so the medication
must be able to enter infected cells to be effective.
Second, viruses are such simple structures that the
choice of which of their chemical processes to attempt
to disrupt is limited. Third, viruses use the host cell’s
DNA and enzymes for self-replication, and a medica-
tion that interferes with DNA or enzymes may kill the
host cell even as it kills the virus.
These problems are illustrated by zidovudine
(AZT), the first medication that was effective against
HIV, the virus that causes AIDS. Zidovudine works by
interfering with DNA synthesis, which the virus must
carry out to reproduce. The side effects of zidovudine,
which are experienced by a significant number of
AIDS patients, are caused by the disruption of DNA
synthesis in the person’s own cells.
Despite these obstacles, successful antiviral drugs
have been developed. Acyclovir, for example, has
proved to be useful in the control (not cure) of herpes
viruses. Ribavirin has been quite effective in the treat-
ment of respiratory syncytial virus pneumonia in
infants and young children. This is an area of intensive
research, and more antiviral medications will
undoubtedly be found within the next decade.
We must be aware, however, that many viruses
undergo mutations with great frequency. Influenza
viruses do (this is the reason we need a new flu vaccine
every year), and it was perhaps inevitable that flu virus
mutations would provide them with resistance to
antiviral medications. In January 2006 the CDC
advised physicians not to prescribe amantadine and
rimantadine for patients with influenza, because more
than 90% of the strains of influenza A virus had devel-
oped resistance to both drugs. Staying one step ahead
of such viruses may prove to be difficult.
FUNGI (SEE TABLE 22–5)
Fungi may be unicellular, such as yeasts, or multicel-
lular, such as the familiar molds and mushrooms. Most
fungi are saprophytes(also called saprobes); that is,
they live on dead organic matter and decompose it to
recycle the chemicals as nutrients. The pathogenic
fungi cause infections that are called mycoses(singu-
lar: mycosis), which may be superficial or systemic.
Yeasts (Fig. 22–4) have been used by people for
thousands of years in baking and brewing. In small
numbers, yeasts such as Candida albicansare part of the
resident flora of the skin, mouth, intestines, and
vagina. In larger numbers, however, yeasts may cause
superficial infections of mucous membranes or the
skin, or very serious systemic infections of internal
organs. An all-too-common trigger for oral or vaginal
yeast infections is the use of an antibiotic to treat a
bacterial infection. The antibiotic diminishes the nor-
mal bacterial flora, thereby removing competition for
the yeasts, which are then able to overgrow. Yeasts may
also cause skin infections in diabetics, or in obese peo-
ple who have skin folds that are always moist. In recent
years, Candidahas become an important cause of noso-
comial infections. The resistance of hospital patients is
often lowered because of their diseases or treatments,
and they are more susceptible to systemic yeast infec-
tions in the form of pneumonia or endocarditis.
Another superficial mycosis is ringworm (tinea),
which may be caused by several species of fungi (see
Table 22–5 at the end of this chapter). The name
ringwormis misleading, because there are no worms
involved. It is believed to have come from the appear-
ance of the lesions: circular, scaly patches with red
dened edges; the center clears as the lesion grows.
Athlete’s foot, which is probably a bacterial-fungal
infection, is perhaps the most common form of ring-
worm.
The systemic mycoses are more serious diseases
that occur when spores of some fungi gain access to
the interior of the body. Most of these fungi grow in a
mold-like pattern. The molds we sometimes see on
stale bread or overripe fruit look fluffy or fuzzy. The
fluff is called a mycelium and is made of many thread-
like cellular structures called hyphae. The color of a
mold is due to the spore cases (sporangia) in which the
reproductive spores are produced (see Fig. 22–4).
Each spore may be carried by the air to another site,
where it germinates and forms another mycelium.
Because spores of these fungi are common in the
environment, they are often inhaled. The immune
responses are usually able to prevent infection and
healthy people are usually not susceptible to systemic
mycoses. Elderly people and those with chronic pul-
monary diseases are much more susceptible, however,
and they may develop lung infections. The impor-512 An Introduction to Microbiology and Human Disease