Answer: Yes, sometimes.
Whether or not your case of food poisoning is contagious depends on what type of food poisoning you have. (For more information on the types of food poisoning please read "What is food poisoning?")
The viruses, bacteria, and parasites that cause gastroenteritis are often food-borne and are considered to be types of food poisoning. They are contagious, so food poisoning from them is contagious. Food poisoning from Salmonella1, Campylobacter2, E.coli3,and norovirus4 are contagious to other people. When you are sick with one of these illnesses, your vomit and feces are full of the organism and can infect other people. These illnesses don't usually show up for at least 24 hours after you've eaten the contaminated food so you might not attribute the illness to the food.
Another type of food poisoning is from bacterial toxins and is not contagious to other people. It occurs when food has been left out at room temperature or in the temperature danger zone (41°F -140°F) for too long. Bacteria such as Staphylococcus aureus5 and Bacillus cereus6 like to grown on the food and produce toxins. It is the toxins that make you sick. Even if you reheat the food, the bacteria will die but the toxins will still be there and will still make you sick. This type of food poisoning usually strikes with abdominal cramps and vomiting 30 minutes to 7 hours after eating the contaminated food. Once you get "rid" of everything, you usually feel much better quickly.
It is important to try to determine if your illness is contagious to other people and to try to minimize the spread if it is. Please continue reading to help you determine if you have a contagious or non-contagious illness. Do I have the stomach flu or food poisoning?
--Annie Pryor, Ph.D.