A parasite has sickened another 36 people, bringing the outbreak total to 315 patients in 15 states as of Thursday night, according to the Centers for Disease Control and Prevention. At least 18 people have been hospitalized with the stomach bug, the agency said.

Cyclospora, the one-celled parasite that causes diarrhea, stomach cramps and other symptoms normally associated with a viral stomach bug, is common in tropical regions like Latin America, but isn't typically seen in the United States, according to the CDC. Most of the people affected were in Iowa, Nebraska and Texas. New Jersey, Minnesota and Ohio are the latest states to report cyclospora cases.

"CDC really has their hands full with this one," said Dr. Richard Besser, chief health medical correspondent for ABC News. "It's hard to pick up so there's probably a lot more of this disease out there."

Because the illness doesn't spread from person to person, it's possible it came from contaminated food or water, said Dr. Nicole Bouvier, an infectious diseases professor at the Icahn School of Medicine at Mount Sinai in New York City. It's curable.

Cyclospora can seem like a stomach bug, but instead of going away within a few days, it hangs around. If you think you have it, Besser suggested alerting your doctor because he or she can do specific tests and prescribe an antibiotic that will rid your body of the microscopic pest.

Bouvier said, "Doctors can diagnose it by looking for either the parasite itself or eggs of the parasite in a stool sample. They have to do it with a microscope because it's tiny. You can't see it with your naked eye. It's not like a worm."

People with deficient immune systems should be extra careful, but it doesn't transmit person-to-person because it doesn't live on hands, Bouvier said. Therefore, if a whole family has cyclospora, it's because they ate or drank the same thing, not because they infected each other.

"The CDC has to find a connection between these people and, so far, they haven't figured out what links all these people together," Besser said.

The CDC and the U.S. Food and Drug Administration are investigating the outbreak.