The microorganism responsible for typhoid fever is a member of one of the largest and most widespread families of bacteria on Earth with over 1,700 serotypes recognized. The salmonellae are rod-shaped bacteria that have a cell wall and flagella, which give the bacterium motility.

Salmonellae can colonize the gastrointestinal tract of a broad range of animal hosts including mammals, birds, reptiles, amphibians, fish, and insects. Some types of salmonellae are highly adapted to specific animals; others have a wide range of hosts. Because of this versatility and the enormous consequent animal reservoir, the eradication of all salmonellosis would be essentially impossible.

Salmonellosis is generally a mild disease in humans, characterized by a few hours or days of vomiting and diarrhea (gastroenteritis), followed by weeks to months during which the organism is shed asymptomatically in the feces. The disease is usually acquired by ingestion of foods that are contaminated with the organism, but other routes - person-to-person, animal-to-person - may sometimes play a role. In the 1970s in the United States, more than 10 percent of the nation's salmonellosis was acquired from baby turtles, a favored pet of children in those days.

Almost unique among the salmonellae, the typhoid bacillus is adapted to human beings alone. S. typhi also possesses a protective envelope, called the "virulence antigen" or Vi antigen, which appears to help the organism resist the immunologic defenses of the host. The exclusive adaptation of S. typhi to human beings makes control possible through public health measures.

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