These are commonly called food poisoning and cause problems either due to toxins presented to the gut by the bacteria or they are caused by viruses such as Rotavirus in infants or they are caused by parasites such as Giardia. They also can unmask disease such as inflammatory bowel disease and can cause persistent effects such as post-infective diarrhoea which may either be due to the infections continuing or the effect of the infection causing temporary damage to the small bowel and subsequent sensitisation to things like dairy produce and then this leading on to a decrease in the enzymes such as lactase. This has the effect of causing lactose not to be well absorbed and then diarrhoea ensues. Gut infections are usually self-limiting and do not require antibiotics except if they are severe such as amoebic dysentery.