context: Getting sick from eating food that has germs, viruses, or parasites is more common than you might think. An estimated 48 million Americans, that's 1 out of every 6, come down with food poisoning every year. Most get better on their own without medical treatment. You may have symptoms like nausea, vomiting, and diarrhea within hours of eating. But sometimes the symptoms can take days or more than a week to show up. That can make it hard to know if it's food poisoning or something else. The delay also makes it tricky to trace the illness back to the specific food or drink. The same food can affect people differently. Some may feel unwell after just a few bites. Others can eat a lot and have no reaction at all. Food poisoning is both more common and riskier for people with weakened immune systems, infants and young children, pregnant women, and the elderly. Food poisoning can be caused by bacteria, viruses, or parasites. They can exist in foods at any stage, such as when they're growing, packaged, shipped, stored, or cooked. Certain foods are more likely to harbor harmful agents. These include raw eggs, unpasteurized milk and juice, soft cheeses, and raw or undercooked meat or seafood. Fresh produce is another risk. Foods made in bulk are problematic, too. A single bad egg could affect the whole batch of omelets in a buffet. You could make trouble for yourself by not washing the cutting board or your hands as you prepare different foods. Your chances of getting food poisoning are higher in the summer. In 90-degree heat, food can start to spoil within an hour. At a picnic or during a camping trip, you are more likely to eat undercooked grilled meats or to handle raw meat without access to soap and water. Bacteria can grow quickly inside tepid coolers. So if you're picnicking on a hot day, put leftovers back in with fresh ice. In 4 out of 5 cases of food poisoning, you never find out exactly what caused it. That's OK because you most likely will get better on your own. But in cases where the culprit is found, it's usually one of the following: Norovirus , often called stomach flu, is behind more than half of the foodborne illnesses in the U. S. where the cause is known. Norovirus can sicken you not only through eating contaminated foods, but also through touching doorknobs and other surfaces or having contact with an infected person. You should wipe down the kitchen if someone in your house has it. It typically takes 12-48 hours before you feel sick. Your symptoms may last 1-3 days. Salmonella is the name of a group of bacteria. They grow in undercooked eggs and meat. But you can also get salmonella from unpasteurized milk or cheese. Some fruits and vegetables, such as melons or sprouts, can also cause it. Symptoms start within 1-3 days and can last up to a week. Clostridium perfringens are bacteria that are more likely to show up when foods are prepared in bulk, such as in cafeterias or nursing homes or for catered events. Cooking kills the bacteria but not its spores. So food left warming can grow new germs. You can get it from beef, chicken, or gravy. You may have cramps and diarrhea but no other symptoms. You get sick within 6-24 hours and are usually feeling better in a couple of days. Campylobacter comes from undercooked poultry, unpasteurized milk, and sometimes water. It may take 2-5 days to develop symptoms you can notice. But you should feel better in another 2-10 days. You can't pass it to anyone. But if it's serious, you might have bloody diarrhea. Some bacteria cause fewer cases of food poisoning but can make you very sick. They can even cause death. They include: E. coli. This is the name of a type of bacteria found in the intestines of animals. You can get this from undercooked ground beef, unpasteurized milk, sprouts, or any food or liquid that has had contact with animal feces or sewage. Some strains are harmless. Others can make you very sick. Listeria is an unusual bacterium that can grow in cold temperatures such as in the refrigerator. It's found in smoked fish, raw (unpasteurized) cheeses, ice cream, pates, hot dogs, and deli meats. Pregnant women and others with weakened immune systems, can feel sick from milder infections from listeria within a day. Other people with a more serious listeria infection called listeriosis may not show symptoms for a week or even a couple of months. In addition to diarrhea and vomiting, listeria can cause unusual symptoms, including weakness, confusion, and a stiff neck. It can also be deadly. If you have a stiff neck with a fever, you may need antibiotics. If you think you may have food poisoning, talk to your doctor.