Nearly any food, but seafood, produce, poultry, beef and eggs are the top five culprits. Raw foods and undercooked meats and seafood, as well as inadequately refrigerated foods, are riskiest, since refrigeration retards the growth of bacteria, and cooking at high temperatures kills most of them. Home-cooked meals are at least as risky as restaurant food.
You’ll probably never know unless you go to a doctor and have a stool test. There are more than 200 foodborne infections, caused by bacteria (notably Campylobacter, Salmonella, Staphylococcus, and E.coli), viruses, and parasites. Toxins in the food (such as mushroom toxin and toxins from some fish) can also cause 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.
The incubation period can be anywhere from one hour to several weeks (the latter is rare). For the most common infections, it takes 4 to 48 hours for symptoms to appear. For parasites it can take more than a month. The longer the time lag, the harder it is to figure out the cause. So don't assume it was your most recent meal that did it.
No. The microorganisms that cause spoilage are different from those that cause food poisoning, which seldom produce perceptible changes in food. Thus, food that looks and smells okay is just as likely to make you sick as food that looks spoiled.
There is no such thing as stomach flu; the influenza virus doesn't cause diarrhea and vomiting in adults. But it may well be an intestinal virus, such as rotavirus or norovirus. These "stomach bugs" are highly contagious and travel in feces; food preparers or handlers who don't wash their hands thoroughly after using the bathroom are the most common source of outbreaks. You get infected from contaminated food or water or from touching contaminated objects and then touching your mouth or nose.
In some cases, foods or organisms are different today, according to the Institute of Food Technologists. Foods may contain new organisms, and organisms can evolve to become more virulent. In any case, people back then did get sick, but they usually didn't know it was food poisoning. Food is much safer today, thanks in large part to preservatives and better knowledge about food handling and preparation.
If you have bloody stool, fever, severe abdominal pain, or prolonged or severe vomiting or diarrhea, or are very dehydrated, you should see a doctor. The elderly, young children, pregnant women, and anyone with a weakened immune system should seek medical attention even for milder symptoms.