What could cause severe diarrhea from eating something?

E. coli, rotavirus: Enterotoxigenic e. Coli is probably the most common bacterial cause, the rotavirus family is probably the most common viral cause and g. Lamblia is probably the most common parasitic cause.

Food poisoning: In addition to numerous true infections with bacteria, viruses, parasites there is the ingestion of preformed toxins made by bacteria growing in the food (staph aureus, clostridium perfringens, etc.).