What are the causes of food borne illnesses?

Bacteria/toxins. Most are due to eating a toxin produced by a bacteria e.g. Staph (causes fever, lots of vomiting, diarrhea), or ingesting a dose of bacteria itself, e.g. Salmonella, campylobacter. Foods contaminated by fecal or disease causing bacteria (even organic), undercooked to kill bacteria present, served by handlers with poor hygiene (eew) transmit disease. Bad water, hygiene spreads parasite, viral infect.
Food. In appropriately washed, heated, boiled or cooked food, food eaten more than 4 hours after being outside etc.