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. This
may be followed by severe agitation and
delirium, leading to coma and death.
Vaccination confers long-lasting immu-
nityand 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 1
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
antibodiesto 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 10 per cent of victims die.
yersinia A class of bacteria containing
the organism responsible for the bubonic
plague (YERSINIA PESTIS). In other forms,
yersinia is responsible for a variety of
infections, such as gastroenteritis, par-
ticularly in young children, and arthritis
and septicaemiain adults.
yin and yangThe 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
of macrobiotics.
yogaA 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 adoptsa 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
disorders, hypertension (high blood
pressure), and glaucoma(increased pres-
sure in the eye).
yolk sacThe membranous sac, other-
wise known as the vitelline sac, that lies
against, and is attached to, the front of
theembryo 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 umbilicus. The
yolk sac is believed to assist in the trans-
portation of nutrients from the mother
to the early embryo.
yttriumA very rare metal that, in its
radioactive form, is sometimes used in
cancer therapy and to treat joints affec-
ted by arthritis.YERSINIA YTTRIUM
Y