What number of bugs = illness?
It depends on the organism. With some types it can take 100 million organisms, but with very virulent ones, a few organisms can cause illness. Viruses generally have a low infectious dose, and they multiply much faster than bacteria. It also depends on the age and health of the person eating the food. People with impaired or underdeveloped immunity, such as the elderly, young children, pregnant women, and those with certain chronic diseases (such as cancer or HIV), can become sick from smaller doses. They are also at higher risk for serious complications.