Bacteria and viruses are the most common cause of food poisoning while the severity may vary depending on the which bacterium or virus has contaminated the food.
Bacteria and Viruses that commonly cause food poisoning are:
Bacillus cereus
Botulism
Campylobacter
Clostridium perfringens
E. coli
Hepatitis A
Listeria
Norovirus (Norwalk Virus)
Salmonella
Shigella
Staphylococcus