Tumor viruses WORLD OF MICROBIOLOGY AND IMMUNOLOGY558•
tularensis(type A). The pneumonia can progress to respiratory
failure and death. The symptoms typically tend to appear three
to five days after entry of the microbe into the body.
The infection responds to antibiotic treatment and
recovery can be complete within a few weeks. Recovery pro-
duces a long-term immunityto re-infection. Some people
experience a lingering impairment in the ability to perform
physical tasks. If left untreated, tularemia can persist for
weeks, even months, and can be fatal. The severe form of
tularemia can kill up to 60% of those who are infected if treat-
ment is not given.
Avaccineis available for tularemia. To date this vaccine
has been administered only to those who are routinely exposed
to the bacterium (e.g., researchers). The potential risks of the
vaccine, which is a weakened form of the bacterium, have
been viewed as being greater than the risk of acquiring the
infection.See alsoBacteria and bacterial infection; Bioterrorism, pro-
tective measures; Infection control; ZoonosesTTumor virusesUMOR VIRUSES
Tumor virusesare those viruses that are able to infect cells
and cause changes within the cell’s operating machinery such
that the cell’s ability to regulate its growth and division is
destroyed and the cells become cancerous.
Human papillomavirus, hepatitisB, Epstein-Barr virus,
human T-cell leukemia virus, SV-40, and Rous sarcoma virus
are all tumor viruses.
The ability of the Rous sarcoma virus to cause sarcomas
(cancers of connective tissue) has been known since 1911,
when Peyton Rousdemonstrated that a sarcoma material from
chicken could be filtered and the filtered fluid was still capa-
ble of inducing the cancer. The virus was both the first onco-
genic (cancer-causing) virus to be discovered and (although
not known until much later) the first retrovirus to be discov-
ered. Another, well-known example of a retrovirus is HIV.
There are some 90 types of human papillomavirus,
based upon the genetic sequence of their genomes. The target
of the viral infection is a certain type of epithelial cell known
as stratified squamous epithelium. The cells can be located on
the surface of the skin, or can be mucosal cells in regions of
the body such as the genital tract. For example, two human
papillomaviruses are the most common cause of genital warts.
While these warts are noncancerous, other types of papillo-
mavirus result in the development of cervical cancer.
Furthermore, human papillomavirus types 16 and 18 are the
main cause of genital tract malignancies. The virus is trans-
mitted from person to person typically via sexual contact.
How human papillomavirus triggers the uncontrolled
growth that is a hallmark of cancerous cells is still unknown.
Studies have determined that in cells that have not yet become
cancerous, the viral genetic material is not associated with the
cell’s genetic material, and that production of new virus parti-
cles is still occurring. However, in cancerous cells the viral
genetic material has been integrated into that of the host andno new virus particles are being made. Whether the integration
event is a trigger for cancerous growth is not known.
The hepatitis B virus is associated with liver damage
and liver cancer. The virus is transmitted from person to per-
son via contaminated blood (which commonly occurs via
sharing of needles), breast milk, and possibly saliva. Over
90% of all hepatitis B infections are cleared as the immune
systemresponds to the infection. However, in some 5% of
those infected the infection becomes chronic. Infected indi-
viduals can be asymptomatic, but remain carriers of the virus
and thus able to pass on the virus to others.
Chronic infection with the hepatitis B virus greatly ele-
vates the chances of developing cancer of the liver. Because
the virus can be present for decades before the damage of liver
cancer is diagnosed, the best strategy is the preemptive use of
hepatitis B vaccinein those offspring born to mothers who are
known to be positive for the virus.
As for human papillomavirus, the molecular mechanism
by which hepatitis B virus triggers cancerous growth of cells
is unknown. The periodic response of the immune system to
the virus may over time favor the expression of genes whose
products are involved in overriding growth and division con-
trol mechanisms.
The Epstein-Barr virus is linked to two specific cancers.
One is called Burkitt’s lymphoma, a cancer of the B-cell com-
ponents of the immune system. The lymphoma is a common
cancer of children and occurs almost exclusively in the central
region of Africa. The region’s high rate of malariamay play a
role in the prevalence of the lymphoma, as malaria causes an
increase in the number of the already-infected B-cells. The
rapid increase in the virally infected B-cells might cause a
genetic malfunction that leads to tumor development. The sec-
ond cancer associated with the virus is nasopharyngeal carci-
noma. This cancer is restricted to the coastal region of China,
for as yet unknown reasons.
Human T-cell leukemia virus causes cancer in the T-cell
components of the immune system. Infection is widespread in
Japan and areas of Africa, and is spreading to western nations
including the United States.
A virus designated SV-40, which is harbored by
species of monkeys, is isolated from a sizable number of
cancer sufferers.See alsoOncogene; Oncogenetic researchTTyphoid feverYPHOID FEVER
Typhoid fever is a severe infection causing a sustained high
fever, and caused by the bacteriaSalmonella typhi—similar to
the bacteria spread by chicken and eggs resulting in
“Salmonella poisoning,” or food poisoning. S. typhibacteria,
however, do not multiply directly in food, as do the
Salmonellaresponsible for food poisoning, nor does it have
vomiting and diarrhea as the most prominent symptoms.
Instead, persistently high fever is the hallmark of infection
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