The diarrhea is usually caused by opportunistic infections of the digestive tract; the most common disease agents are Cryptosporidium parvum, Giardia lamblia, Campylobacter, or rotaviruses.
A variety of bacteria cause food poisoning, including Salmonella, Staphylococcus aureus, Campylobacter jejuni, Escherichia coli, Shigella, and Clostridium botulinum.
Campylobacter is the leading cause of bacterial diarrhea worldwide, responsible for more cases (2 million or more) of bacterial diarrhea in the United States than Shigella and Salmonella combined.
Campylobacter is carried by healthy cattle, chickens, birds, and flies.
Although eating chicken is a known risk factor, drinking water and eating salads have not been considered significant risks until studies of causes released in 2003 showed possible association with Campylobacter diarrheal infections.