Parasitic organisms—such as certain worms and microscopic protozoa—survive by living in or on a host. Humans can become hosts unintentionally, most often through contaminated food or water. Once inside the body, some parasites remain in the digestive tract, while others can migrate to muscles or organs. Symptoms often develop gradually and may resemble common conditions like fatigue, mild digestive upset, or anemia, which is why infections can go unnoticed for long periods.Undercooked meat is one of the more common routes of exposure worldwide. Pork, beef, and fish can carry parasites that are not visible to the eye. Pork that is eaten raw or insufficiently cooked may transmit organisms capable of forming cysts in tissues. Beef prepared rare or raw can occasionally harbor tapeworms that attach to the intestine and interfere with nutrient absorption over time.

Fish presents additional considerations, particularly when consumed raw or lightly cured. Certain parasites adapted to marine environments can irritate the stomach lining, causing abdominal pain, nausea, or vomiting. In some cases, the body reacts with allergy-like symptoms rather than a typical infection, which can make diagnosis less straightforward. Commercial freezing standards reduce risk significantly, but improper handling or preparation can undermine those protections.Shellfish deserve special attention because they filter large volumes of water. When harvested from polluted areas, they can accumulate parasites present in contaminated water. Eating shellfish raw allows these organisms direct entry into the body. Thorough cooking—until the flesh is fully opaque—is one of the most reliable ways to reduce risk.