These bacteria inhabit the gastrointestinal tracts of a large variety of species. Water trough sediments, soil, and animal bedding can also harbor O157 and DT104. This wide range of habitats and reservoirs, ranging from stagnant ponds to insect guts to domestic pet intestines, makes complete eradication of these pathogens very unlikely. Animals infected with these bacteria excrete them in feces. This feces is then transferred by insects, water, or other contact to food products.
Humans ingest the fecal material, and the bacteria along with it.
We call this the fecal-oral route of infection. Generally, an adult human must ingest a large number of bacteria to cause clinical disease. The actual number varies between bacteria. The infectious dose of Salmonella is between 1,000,000 and 10,000,000,000 organisms. The infectious dose of E. coli O157 can be as low as 10 organisms.