of capillaries. Its surface is covered with
hundreds of hairlike structures (micro-
villi). The villi and microvilli provide a
large surface area for absorption of food
molecules from the intestine into the
blood and the lymphatic system.
vinca alkaloids A group of substances
derived from the periwinkle plant (VINCA
ROSEA) that are used to treat leukaemias,
lymphomas, and some solid tumours,
such as breast cancerand lung cancer.
All vinca alkaloids can cause neurological
toxicity, which appears as neuropathy.
Other side effects may include abdomi-
nal pain, constipation, and reversible
alopecia. Common vinca alkaloids are
vinblastine, vindesine, and vincristine.
Vincent’s diseaseA severe form of
gingivitis in which bacterial infection
causes painful ulceration of the gums.
(See also gingivitis, acute ulcerative.)
vincristineA vinca alkaloidused to treat
certain cancers. One particular side effect
of vincristine is peripheral or autonomic
neuropathy; but, unlike the other vinca
alkaloids, it causes very little reduction
in blood-cell production by the bone
marrow. Other side effects may include
abdominal pain, constipation, and re-
versible alopecia.
viraemiaThe presence of virus particles
in the blood. Viraemia can occur at cer-
tain stages in a variety of viral infections.
Some viruses, such as those responsi-
ble for viral hepatitis, yellow fever, and
poliomyelitis, are transported in the
bloodstream. Others, such as the rubella
virus and HIV, multiply in, and spread
via, certain white blood cells. If viraemia
is a feature of a viral infection, there is a
risk that the infection may be transmitted
to other people in blood or blood prod-
ucts, or by insects that feed on blood.
viral haemorrhagic feverDiseases
that are prevalent in Africa and cause
severe bleeding. There are several types,
including Ebola fever, Lassa fever, Han-
tavirus, and Marburg fever. The diseases
are fatal in a large percentage of cases,
but Lassa fever may respond to antiviral
drugsif given in the 1st week.
virginityThe physical state of not hav-
ing experienced sexual intercourse.
virilismThe presence in a woman of
masculine characteristics. Virilism iscaused by excessive levels of androgen
hormones. Androgens are male sex hor-
mones which, in women, are normally
secreted in small amounts by the
adrenal glands and ovaries. Raised levels
induce various changes in women, in-
cluding hirsutism; male-pattern baldness;
disruption or cessation of menstruation;
enlargement of the clitoris; loss of nor-
mal fat deposits around the hips;
development of the arm and shoulder
muscles; and deepening of the voice.
virilityA term used to describe the
quality of maleness, especially in sexual
characteristics and performance.
virilizationThe development in a woman
of male characteristics as a result of
overproduction of androgen hormones
by the adrenal glands and/or ovaries.
This may be due to various conditions
such as certain adrenal tumours, poly-
cystic ovary (see ovary, polycystic) and
some other ovarian cysts, or congenital
adrenal hyperplasia.
virionA single, complete, virusparticle.
virologyThe study of virusesand the
epidemiologyand treatment of diseases
caused by viruses. In a more restricted
sense, virology also refers to the isola-
tion and identification of viruses to
diagnose specific viral infections. Depend-
ing on the type of virus, this may involve
growing viruses in cultures of human or
animal cells, staining or microscopic
examination of specimens containing
viruses, or immunoassaytechniques.
virulenceThe ability of a microorganism
to cause disease. This can be assessed
by measuring what proportion of the
population exposed to the microorgan-
ism develops symptoms of disease, how
rapidly the infection spreads through the
body, or the mortality from the infection.
virusesThe smallest known types of
infectious agent. It is debatable whether
viruses are truly living organisms or just
collections of molecules capable of self-
replication under specific conditions.
Their sole activity is to invade the cells
of other organisms, which they then
take over to make copies of themselves.
Outside living cells, viruses are inert.
A single virus particle (virion) consists
of an inner core of nucleic acid,which
may be either DNAor RNA, surroundedVINCA ALKALOIDS VIRUSESV