tuberculosis(TB) A contagious bacterial infection
caused by bacteria that are members of the Mycobac-
teriumgenus, usually by Mycobacterium tuberculosis,
that usually affects the lungs or other body organs. Can
bespread by person-to-person contact through watery
airborne droplets of coughs and in the mucus that
infected persons cough up from their throats. Some 10
to 20 million Americans have been infected, and one-
third of the world’s population. Can be treated with
medicines, and a vaccine is available.
tularemia An infectious disease with high fever and
other complications caused by the bacterium Pasteurel-
la tularensis,which affects mostly people who handle
infected wild rabbits, although it can be transmitted by
ticks, contaminated food, water, and other animals.
Mortality rate is about 6 percent.
tumor (neoplasm) Amass of abnormal tissue that
arises from normal tissue; grows abnormally in rate
and structure and serves no physiological function;
abnormal regulation of cell growth.
Tumors can be benign, characterized by being
slow-growing and harmless, depending on the location;
malignant, characterized by being fast-growing, likely
to spread, and capable of damaging other organs or
systems; or intermediate, which is a mixture of benign
and malignant cells.
Tumors can be caused by abnormalities of the
immune system, radiation, genetic mutations, viruses,
sunlight, tobacco, benzene and other mutagens, poi-
sonous mushrooms, and aflatoxins.
Also called neoplasm, the preferred medical term.
tumor suppressor gene Anormal gene whose pur-
pose is to suppress cell growth or proliferation (can-
cer). However, if a mutation, inactivation, or deletion
of part or all of one allele of a tumor suppressor gene
occurs, it predisposes the individual to increased risk of
tumor or cancer development.
tundra An area of flat or undulating treeless barrens
found across the northern parts of Asia, Europe, and
North America, covering over one-fifth of the earth’s
surface. Covered with lichens, sedges and grasses,
mosses, and low shrubs (dwarf willows and birches), it
is characteristic of the Arctic and alpine (high-altitude)
regions. Below the tundra soil is permafrost, a perma-
nently frozen layer of ground. During brief summers,
the top section of the soil thaws long enough for plants
and microorganisms to grow and reproduce. Animals
found in the tundra include the Arctic fox, caribou,
golden eagle, gray wolf, musk ox, Norway lemming,
polar bear, red phalarope, ruddy turnstone, and Eski-
mos and their sled dogs. The average annual tempera-
ture is only 10 to 20°F (–12°C to –6°C), with
temperatures sometimes dipping below –60°F (–51°C)
and the annual precipitation usually less than 10 inches
(25 centimeters).turgid Swollen, rigid, or firm. Cells become turgid
upon entry of water.turgor pressure Like air pressure in a car tire, it is
the outward pressure that is exerted against the inside
surface of a plant cell wall under the conditions of
water flowing into the cell by osmosis, and the result-
ing resistance by the cell wall to further expansion.turtles A group of slow-moving armored reptiles
(subclass Anapsida, order Testudines) with 11 families
that include terrestrial, amphibious, freshwater, and
marine species. They are oviparous, cold-blooded, have
scaly skin, and lay eggs. A turtle’s armor is the back-
bone fused to the shell, and the central disk of the
upper shell, or carapace, is formed by a fusion of bony
plates with the turtle’s ribs. Their ribs lie outside the
shoulder and pelvic bones, which allows them to
retract their limbs and heads into the shells. They live
worldwide except for Antarctica. Turtles are the oldest
living group of reptiles, first appearing about 200 mil-
lion years ago. Many of the terrestrial turtles are called
tortoises. Many species today are endangered because
of habitat destruction.twins Apair of offspring that go through the same
gestation period at the same time from the same
mother. Two forms exist. Identical twins develop fromtwins 331