Cockroaches are primitive creatures which have remained largely unchanged for millions of years.
They will eat virtually anything, food spills, faecal matter, soap, hair, paper etc. Ingested bacteria can survive in the cockroaches digestive system for months.

As cockroaches will vomit and defecate on food, it is though that diseases may be transmitted to humans. The proteins in cockroach saliva and waste can also cause allergies and aggravate asthma.