There are more parasites than any other organism on Earth—which is one of the reasons Emmitt Jolly finds the common but misunderstood creatures so fascinating. “That is the most successful form of life,” says Jolly, a professor of biology at Case Western University. People often assume that honor would go to insects, but “the reality is that every insect you find is parasitized by something.”

Sometimes, humans are too. Nearly 850 species of parasites are known to infect people; some, like the potentially deadly plasmodium parasite that causes malaria, are well-known. Others fly more under the radar.

We asked Jolly and other experts how parasites are transmitted, what it feels like to be infected, and what you can do to protect yourself.

How people get parasites

Parasites truly live u

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