Transmission
Diarrhea is caused by infectious organisms, including viruses, bacteria, protozoa, and helminths, that are transmitted from the stool of one individual to the mouth of another, termed fecaloral transmission. Some are well known, others are recently discovered or emerging new agents, and presumably many remain to be identified. They differ in the route from the stool to the mouth and in the number of organisms needed to cause infection and illness. Among bacteria, the ability to survive stomach acid is an important determinant of the inoculum size required to cause illness. For example, Shigella bacteria are resistant to low pH, and a few thousand organisms suffice, which are readily transferred by direct person-to-person contact or through contamination of inanimate objects, such as a cup. In contrast, bacteria readily killed by acid, such as Vibrio cholerae, require millions of organisms to cause illness, and therefore must first multiply in food or water to an infectious dose. Some pathogens, such as rotavirus, display a sharp host species preference, and others have a broad host range. Among Salmonella bacteria, certain bio-serotypes are adapted to infect animals and pose no threat to humans, and others are adapted to humans and do not infect animals. The majority, however, are not adapted to a specific host and can infect either humans or domestic animals, thus facilitating transmission of these organisms to humans. Less than a dozen of the more than 2,500 individual Salmonella cause the majority of human infections, reflecting the requirement for genes that encode essential virulence factors.
The ability to identify virulence genes and their products has led to new molecular approaches to epidemiology and diagnosis, and undoubtedly will lead to new measures to prevent and treat diarrhea. Molecular methods also allow the separation of organisms that otherwise appear to be identical. Nonpathogenic Escherichia coli in normal stool cannot be separated from diarrhea-causing E. coli by standard methods; however, identification of virulence genes or factors distinguishes five groups of E. coli that cause illnesses ranging from cholera-like watery diarrhea to neonatal diarrhea, persistent diarrhea, and bloody diarrhea (Nataro and Kaper 1998).
