Webster's New World Medical Dictionary

yellow fever vaccination
A live attenuated
(weakened) viral vaccine for yellow fever. Yellow
fever vaccination is recommended for people travel-
ing to or living in the tropical areas in the Americas
and Africa where yellow fever occurs. Because yel-
low fever vaccination is a live vaccine, it should not
be given to infants or people with immune-system
problems.
yellow jacket stings
Stings from yellow jackets
that can trigger allergic reactions of varying severity.
Avoidance and prompt treatment of allergic reac-
tions (including diphenhydramine, brand name:
Benadryl; and epinephrine, brand name: Epi-Pen)
are essential. In selected cases, allergy injection
therapy is highly effective for prevention.
Yersinia
A family of bacteria that includes
Yersinia pestis, which causes the bubonic, pneu-
monic, and septicemic plagues; Y. entercolitica,
which causes intestinal infections, including mesen-
teric lymphadenitis, a condition that mimics appen-
dicitis; and Y. pseudotuberculosis, which usually
adversely affects only animals but can cause illness
in immunocompromised patients. Both Y. entercol-
itica and Y. pseudotuberculosis have also been
implicated in a viral form of arthritis. Infection with
Yersinia bacteria can be treated with antibiotics. See
also 
plague.
Y-linked
A gene on the Y chromosome that is
passed from father to son. See also 
holandric
inheritance.
Y-linked inheritance
See 
holandric inheri-
tance.
yoga
A relaxing form of exercise that was devel-
oped in India and involves assuming and holding
postures that stretch the limbs and muscles, doing
breathing exercises, and using meditation tech-
niques to calm the mind. Yoga appears to have ben-
efits for increasing physical flexibility and reducing
internal feelings of stress. Yoga may be recom-
mended as an alternative or complementary health-
promoting practice.
yogurt
Milk that is fermented with a culture 
of Lactobacillus (the milk bacillus) and often with
acidophilus and other helpful bacteria. See also
probiotic.