A remarkable variety of worms can infect humans in the United States. These worms may be round or flat, short or long, with suckers or with three lips. The same worm species can produce symptoms ranging from none at all or slight fatigue to severe illness, seizures and death in some cases. The difference between ingesting eggs and ingesting immature adults -- known as larvae -- is important, since different diseases may result.

Pinworms

Pinworms, the most common worm infection in the U.S., are white-tan, round and less than half an inch in length. Pinworms can live in the human colon and rectum. They cause itching when the female worm crawls out at night and lays her eggs around the anus. Pinworm infection is transmitted when eggs are ingested through contact with contaminated feces or with clothing or surfaces harboring eggs. Rarely, the worms can infect the female reproductive tract or other sites. Schoolchildren and institutionalized people are at greater risk of developing a pinworm infection.

Tapeworms

Equipped with hooks and suckers that attach to the intestines, tapeworms may grow to be 20 feet long. Ingesting larvae in undercooked pork or beef can transmit tapeworms. The Centers for Disease Control and Prevention reports that intestinal tapeworms are far more common in other countries than in the U.S. All it takes is a single tapeworm, however, to produce thousands of eggs.

Tapeworm Eggs and Larvae

With 50,000 eggs per worm, a pork tapeworm disperses its eggs through human feces. Ingested eggs can become larvae within the body. Larvae invade the muscles and brain, forming cysts that can cause seizures, stroke or death. In the U.S., at least 1,000 people are admitted to the hospital each year because of tapeworm cysts in the brain, according to the CDC. Tapeworm infections are often the result of travel outside the U.S. or exposure to people who have lived or traveled outside the country.

Roundworms From Undercooked Meat

Undercooked pork, poultry and wild meats may harbor larvae of tiny roundworms called Trichinella. In the intestine, the larvae grow into 0.10-inch-long worms, which then squirm through the bloodstream to access muscle. According to the CDC, from 2008 to 2012 some 90 people in the U.S. were reported to have Trichinella. Infections range from producing no symptoms to being fatal.

Roundworms From Dogs and Cats

These worms do not prefer humans, but people become infected through accidental ingestion of eggs from feces-contaminated soil. Less commonly, undercooked meat is to blame. Playgrounds and sandboxes harbor eggs, and children are at higher risk due to their play habits and hygiene. Many more Americans have been exposed than realize it. Larvae penetrate the intestines to enter the bloodstream, potentially affecting vital organs, muscles and eyes. The body battles the larvae, creating damage. At least 70 people go blind each year, according to the CDC, and most of the eye damage occurs once the larvae have already died. Overall, infections range from very mild to very serious.

Other Worms Transmitted From Soil

Although cases of other soil-transmitted worms -- Ascaris, hookworms and whipworms -- are uncommon in the U.S., millions of people have them globally. Human feces as fertilizer and poor sanitation contribute to transmission. Hookworms are about a quarter-inch long, with hooks. Ascaris can be over a foot long, yet symptoms are often mild. Whipworms are about 1.5 inches long and attach to the large intestine via threads.