It is believed that the cockroach may be a reservoir for a range of bacteria including salmonella, staphylococcus and streptococcus. The cockroach can also harbour viruses such as the polio virus.
Like the household fly, the cockroach will eat virtually anything ranging from food spills on a kitchen floor to faecal matter. Ingested bacteria can survive in the cockroach's digestive system, sometimes for months or even years, and are passed in its droppings. Cockroaches will vomit and defaecate on food and it is thought that disease may be transmitted to humans when humans eat food contaminated by cockroaches.
Recent research suggests that the cockroach may also be associated with human allergies.