A sandfly is one of the “criminals” whose misdeeds are explored on “Monsters Inside Me.” (Animal Planet/Animal Planet)

After a family vacation in Colombia, an American teenager returned home with a rash. Doctors found the cause — maggots that were hatching and eating her flesh. In all, 212 tiny larvae were picked from her head.

Her harrowing experience and that of other Americans who hosted foreign organisms make up "Monsters Inside Me," an Animal Planet show whose eighth season is getting underway.

The series is popular because people respond with fear about these invasive creatures, many of which are parasites, said Dan Riskin, a biologist who appears in each episode. But parasites deserve our respect, he said.

Half of the world's organisms are parasites, scientists have estimated, and they have adapted remarkably well. For example, filarial worms — which cause elephantiasis, a lymphatic disease characterized by enlargement of body parts — can live in a human host eluding our immune system and not causing symptoms for years.

Parasites such as the four species of plasmodium that cause malaria in humans infect large populations worldwide and kill hundreds of thousands of people. Many parasitic diseases, such as dengue fever and sleeping sickness, are not common in developed countries, but others exist in the United States. The Centers for Disease Control and Prevention estimates that each year, 1.1 million Americans are infected with Trichomonas vaginalis, which causes the sexually transmitted disease trichomoniasis. And 800,000 people are infected each year with Toxoplasma gondii, which causes toxoplasmosis. Humans get infected "by eating raw or undercooked meat, or ingesting food, soil or water contaminated by cat feces" containing the parasite, according to the CDC. Toxoplasmosis causes hundreds of deaths and thousands of hospitalizations each year, the agency stated.

"Monsters Inside Me" focuses on parasites that are not often seen in the United States. The past season featured Dirofilaria immitis, a parasitic roundworm that infected a woman's eye, and Balamuthia mandrillaris, an amoeba that caused blindness, severe neurological symptoms and ultimately the death of a 6-year-old boy.

The parasites and other foreign organisms and objects are presented as "criminals" in the show, a biological "Law and Order," with physicians and the patient trying to find them and to devise a cure.

"Monsters Inside Me," based on real-life cases, serves as a warning, Riskin said. Because many parasites are uncommon in the United States, Americans aren't as alert to their dangers when they travel abroad. "Read up on what you can catch in the place you are visiting," he advised. That might reduce your chances of becoming infected by a parasite, such as botfly maggots inside your head, as Riskin was during a trip to Belize.

Correction: An earlier version of the article misidentified leprosy as a parasitic disease.

Sofia Anjuman Ali