Most bouts of food poisoning clear up on their own. But severe symptoms—like diarrhea that’s bloody or lasts more than three days, vomiting too much to keep liquids down, a fever over 102ºF, or signs of dehydration (like dizziness or infrequent urination)—require medical help. (If in doubt, call your doctor.) Here’s when and how contaminated food typically makes people sick.

(Scroll down to the table to see the symptoms and consequences of an infection from each bug.)

Meet the enemy

Campylobacter

Responsible for about 1.5 million cases of food poisoning each year in the U.S.

Common food sources

Undercooked poultry, raw milk.

Clostridium botulinum (Botulism)

Very rare, but can be deadly if not treated. Caused by a bacterial toxin that attacks the body’s nervous system.

Common food sources

Improperly home canned foods.

Clostridium perfringens 

Produces heat-resistant spores that can transform into active, toxin-producing bacteria in foods that aren’t kept at the proper temperature. Causes about 1 million cases of food poisoning each year in the U.S.

Common food sources

Improperly cooled or stored high-protein foods, particularly meats and gravies.

Cyclospora

A parasite spread by food or water contaminated with feces, typically in tropical and subtropical regions.

Common food sources

Imported produce like berries, basil, snow peas, or lettuce.

Enterotoxigenic E. coli

Rare in the U.S., but the most common cause of “traveler’s diarrhea” in countries with poor sanitation.

Common food sources

Food or water that has been contaminated by feces.

Shiga toxin-producing E. coli (Including E. coli O157:H7)

A class of E. coli that can cause kidney failure and that may be fatal in severe cases.

Common food sources

Undercooked ground beef, leafy greens, raw sprouts.

Listeria monocytogenes

Rare, but one of the leading causes of death from foodborne illness. Primarily strikes older adults, pregnant people, newborns, and people with weakened immune systems. Can grow at refrigerator temperatures.

Common food sources

Raw milk, cheeses made with raw milk, deli meats.

Noroviruses

Often called the “stomach flu.” Causes 19 to 21 million (typically mild) infections each year in the U.S. Noroviruses spread easily and quickly (including for a few days after you feel better) through person-to-person contact and contaminated food.

Common food sources

Food handled by an infected person, raw produce, raw mollusks (oysters, clams, mussels, scallops).

Salmonella

The leading cause of serious foodborne illness in the U.S. Responsible for roughly 1.4 million cases each year.

Common food sources

Eggs, poultry, beef, flour (and many more foods).

Shigella

Spread through contaminated feces. Makes its way into food usually due to poor personal hygiene by someone preparing the food.

Common food sources

Ready-to-eat foods prepared by an infected food handler, raw fruits and vegetables.

Staphylococcus aureus

Typically caused when food isn’t kept at the proper temperature. Produces a toxin that isn’t destroyed by cooking.

Common food sources

Food contaminated by a person colonized with the bacteria. Frequently caused by improperly refrigerated foods like meats, potato salad, and egg salad.

Vibrios

Found in some warm coastal waters. Infections can happen any time of year, but are most common between May and October, when waters are warmest.

Common food sources

Raw or undercooked shellfish (particularly oysters) and fish.

Yersinia

Infection may be misdiagnosed as appendicitis or Crohn’s disease.

Common food sources

Undercooked pork like chitlins (pig intestines). 

Sources: Centers for Disease Control and Prevention; Food and Drug Administration; J. Glenn Morris, professor of infectious diseases, director of the Emerging Pathogens Institute at the University of Florida, and co-editor of Foodborne Infections and Intoxications.