Here's a link to the
Centers for Disease Control and Prevention website that tells all you might want to know about foodborne outbreaks, plus some things you might not want to know. The CDC is the agency that actually keeps the statistics about foodborne outbreaks.
Most foodborne illnesses have a much longer incubation period than people think. People tend to think about the last food they ate within the last few hours and blame that - even when they have a specific diagnosis of something that takes 12-72 hours after eating the food before symptoms begin.
I was in Public Health for many years and did lots of Foodborne Outbreak investigations. The investigators don't just guess what food was the problem (although a lot of times, people said their doctor had told them he/she 'knew' what food it was by their symptoms; the doctors usually turned out to be wrong).
After questioning sick
and well people to see what they ate, all the information is entered into a computer data base to calculate the odds that any particular food caused the problem. The computer compares the ill people who ate something with the well people who ate the same thing and calculates the odds for each food having been the problem. Not everyone who ate the 'problem food' will get ill, but for a situation where 75 people ate the same thing and only one became ill, the odds that food was the problem are out of the realm of possibility.
Before doing the odds calculations, we usually tried to guess how it would turn out. I remember a wedding where a lot of ill people had eaten chicken and very few well people had. It turned out that the problem was the gravy that many people put on their chicken (figured out by calculating the odds ratio, which was higher for gravy or chicken and gravy than it was for just chicken).
When we had the gravy tested, the spices in it were contaminated. It also turned out the the cooks at the wedding had not followed the gravy cooking directions - it said to start out with cold water and bring it to a boil. They tried to hurry the process by starting with microwaved warm water. When we got the dry gravy mix and the one we cooked using their process tested, both were contaminated with the same germ the patients had. When we cooked a batch according to the product directions, the germs were killed.