Wart : An epidermal tumor of viral origin.

Wassermann test : A complement fixation test used to diagnose syphilis.

Water activity : An expression of the relative availability of water in a substance. Pure water has a water activity of 1.000. The water activity of a solution is one-hundredth its relative humidity.

Water mold : A common term for a member of the division Oomycota.

WB : Immunoblot/Western blot.

WBC : White blood cell (count).

Weil-Felix test : Whole cell agglutination assay that uses Proteus cell surface antigens to detect cross-reactive rickettsial antibodies.

Western blot : Similar to Southern blot, except that antigenic proteins of an organism are separated by gel electrophoresis and transferred to membrane filters. Antiserum is allowed to react with the filters, and specific antibody bound to its homologous antigen is detected using labeled anti-antibody detectors. See immunoblot.

Whey : The fluid portion of milk that separates from curd.

Wheeze : A high-pitched whistling noise made when narrowed airway passages cause difficulty in breathing.

Whipple disease : A disease caused by an uncultured bacterium, Trophyrema whippeli, and characterized by diarrhea, arthritis, and neurologic manifestations.

Whipworm : Common name for roundworm (Trichuris trichiura).

White piedra : A fungal infection (tinea albigena) caused by the yeast Trichosporon beigelii that forms light-colored nodules on the beard and mustache.

Whooping cough (pertussis) : An infectious disease characterized by recurrent spasmodic bouts of coughing that continues until the breath is exhausted, then ending in an inspiratory stridor (the "whoop") caused by laryngeal spasm. It is caused by Bordetella pertussis.

Whorl : A group of cells radiating from a common point.

Widal test (pronounced vidal) : A test involving agglutination of typhoid bacilli when they are mixed with serum containing typhoid antibodies from an individual having typhoid fever; used to detect the presence of Salmonella typhi and S. paratyphi.

Wild type : A strain of microorganism isolated from nature. The usual or native form of a gene or organism.

Wilt : A plant disease in which bacteria invade the vessels of herbaceous plants, interfere with movement of water and nutrients, and produce certain toxins that cause wilting and the eventual death of the plant.

Wine vinegar : Vinegar produced by the oxidation of alcohol in wine by members of the genus Acetobacter.

Winogradsky column : A glass column with an anaerobic lower zone and an aerobic upper zone, which allows growth of microorgansims under conditions similar to those found in a nutrient-rich lake.

Winterbottom's sign : Enlargement of the posterior cervical lymph nodes (African trypanosomiasis).

Wobble : In reference to reading the genetic code, the concept that nonstandard base pairing is allowed between the anticodon and the third position of the condon.

Wort : The filtrate of malted grains used as the substrate for the production of beer and ale by fermentation.