Shigella is very, very infective. One or two shigella organisms are capable of causing infection. With E. coli 0157, it is less than one thousand. With organisms such as salmonella and the enterotoxin-producing E. coli, which causes turista in travelers in Mexico and other countries, you need many more organisms to cause infection, usually in the millions. Campylobacter jejuni is somewhere in between.
Intestinal motility is also a host defense.
Then the submucosal deep invasion is caused by salmonella, Yersinia and Campylobacter. Then there are a group of viruses that can cause gastroenteritis, diarrhea, Rotavirus, parvovirus, Food from animals, especially poultry, are heavily contaminated with salmonella and Campylobacter. These are natural organisms found in the guts of chickens, pigs, cows and many other animals, but particularly in poultry. Day care is an incubator for Rotavirus, Giardia, Cryptosporidium and in fact, practically anything that is communicable. When there are no fecal leukocytes, there is no blood and there is no mucus, we think of enterotoxin.