more serious cases, the fever is higher
and there is severe headache and pain
in the neck, back, and legs. Damage may
occur rapidly to the liver and kidneys,
causing jaundice and
kidney failure.
may be followed by severe agitation and
delirium, leading to coma and death.
Vaccination confers long-lasting immu-
nity and should always be obtained
before travel to affected areas. A single
injection of the vaccine gives protection
for at least 10 years. Reactions to the
vaccine are rare and are usually trivial,
although children under the age of
should not be vaccinated.
During yellow fever epidemics, diag-
nosis is simple. A diagnosis can be
confirmed by carrying out blood tests to
isolate the causative virus or to find
to the virus.
No drug is effective against the yellow
fever virus; treatment is directed at
maintaining the blood volume. Transfu-
sion of fluids is often necessary. Many
patients recover in about 3 days and, in
mild to moderate cases, complications
are few. Relapses do not occur and one
attack confers lifelong immunity. Overall,
however, about
per cent of victims die.
yersinia A class of bacteria containing
the organism responsible for the bubonic
(yersinia pestis).
In other forms,
yersinia is responsible for a variety of
infections, such as
, par-
ticularly in young children, and
in adults.
yin and yang The 2 opposing and
interdependent principles that are funda-
mental to traditional
Chinese medicine
and philosophy. Yin is associated with
the female, darkness, coldness, and
quiescence; yang embodies qualities of
maleness, brightness, heat, and activity.
In a healthy body, yin and yang are in
balance. The concepts of yin and yang
are also central to the theoretical basis
yoga A system of Hindu philosophy and
physical discipline that is becoming
increasingly popular throughout the
world. The main form of yoga that is
practised in the West is hatha-yoga, in
which the follower adopts a series of
poses, called asanas, and uses a special
breathing technique. The practice of
yoga maintains flexibility of the body,
teaches both physical and mental con-
trol, and is a useful
relaxation technique
If attempted by people in poor health,
or practised incorrectly, yoga may pose
certain health hazards, such as back
pressure), and
(increased pres-
sure in the eye).
yolk sac The membranous sac, other-
wise known as the vitelline sac, that lies
against, and is attached to, the front of
during the early stages of its
existence. During development, the sac
decreases proportionately in size to the
body, reducing finally to a narrow duct
that passes through the
. The
yolk sac is believed to assist in the trans-
portation of nutrients from the mother
to the early embryo.
yttrium A very rare metal that, in its
radioactive form, is sometimes used in
cancer therapy and to treat joints affec-
ted by arthritis.
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