: an infective agent or material contaminated with an infective agent
a: the state produced by the establishment of an infective agent in or on a suitable host b: a disease resulting from infection :infectious disease
: an act or process of infecting <syphilis infection is chiefly venereal>; also: the establishment of a pathogen in its host after invasion
Invasion of the body by various agentsincluding bacteria, fungi (seefungus), protozoans, viruses, and wormsand its reaction to them or their toxins. Infections are called subclinical until they perceptibly affect health, when they become infectious diseases. Infection can be local (e.g., an abscess), confined to one body system (e.g., pneumonia in the lungs), or generalized (e.g., septicemia). Infectious agents can enter the body by inhalation, ingestion, sexual transmission, passage to a fetus during pregnancy or birth, wound contamination, or animal or insect bites. The body responds with an attack on the invader by leukocytes, production of antibodies or antitoxins, and often a rise in temperature. The antibodies may result in short-term or lifelong immunity. Despite significant progress in preventing and treating infectious diseases, they remain a major cause of illness and death, particularly in regions of poor sanitation, poor nutrition, and crowding.