Ulcer: An open sore on the skin or mucous membrane characterized by the disintegration of the tissue and often the discharge of pus (leishmaniasis, amebiasis, Dracunculus infections).

Unilocular: Cyst containing only on cavity.

Uniseriate: With reference to the genus Aspergillus, phialide form directly on the vesicle; a biseriate phialide is supported by a metula.

Universal precautions: See under Isolation, Universal blood/body fluid precautions.

Upper respiratory tract: The nasopharynx, oral cavity, and throat.

Upstream position: Refers to nucleic acid sequences on the 5'-side of a given site on a DNA or RNA molecule. Compare with downstream position.

Urease: An enzyme that catalyzes the decomposition of urea, notable examples being Bacteroides ureolyticus, Clostridium sordellii, and Peptostreptococcus tetradius, which are urease-positive, and two species of Actinomyces (naeslundii and viscosus) which are variable for urease production.

Urethritis: Inflammation of urethra, the canal through which urine is discharged (e.g., gonococcal urethritis).

URI: Upper respiratory tract infection.

Urticaria: An allergic reaction seen on the skin characterized by the development of wheals and intense itching and burning sensations; onset and disappearance are often sudden (filarial infections).

UTI: Urinary tract infection.

Uveitis: Inflammation of the uvea (pigmented, vascular layer of the eye) (Acanthamoeba infections).

UVGI: Ultraviolet germicidal irradiation.