Enterobacteria are a large heterogeneous group of gram-negative rods whose natural habitat is the intestinal tract of humans and animals. The family includes many genera, of which salmonella is one.

There are 1500 to 2000 types of salmonellae, with one type, S. typhi (typhoid fever), notorious, and another type, S. typhimurium, the most common enterocolitis (gastroenteritis) pathogen in the US.

Infections with most other types of salmonella, except for S. paratyphi, derive from environmental sources, principally poultry and livestock. Despite the frequency with which these organisms cause acute gastrointestinal illness, there are remarkably few documented examples of person-to-person spread. An outbreak in a day-care facility was associated with an uncertain number of secondary cases, and long-term surveillance of 54 permanent carriers of nontyphoidal salmonella identified 10 instances of transmitted infection.