Who among us leaves childhood without singing this ghoulish schoolyard refrain, “The worms crawl in, the worms crawl out, in your stomach and out your mouth”?

The morbid yuck factor that makes that ditty so enduring features prominently in “Monsters Inside Me” (8 p.m., Animal Planet), an up-close-and-poisonous look at parasitic infections — and the nasty creatures behind them.

Every episode of “Monsters” unfolds with a series of vignettes. A human falls ill and then his doctors do a little detective work and discover an outbreak traced to parasites either local or imported. Special effects show how the invaders pierce blood vessels, fight off antibodies and even penetrate the brain. Think of it as an episode of “House” without all the campy psychodrama and acerbic misanthropy.

In one particularly gruesome tale, a Chicago medical student falls ill after playing a pick-up game of basketball. At first he believes that he’s just out of shape. Then he suspects a flu. But after unceasing headaches leave him wishing he would simply die, his doctors discover that he’s been infiltrated by something called the Rat lungworm, a creature as nasty as its name.

You can’t say that the narrators of “Monsters” don’t have some sympathy for the monsters. Most parasites have no interest in infiltrating people. And when they do enter the human body, it’s bad news for both invader and host.

As fate and nomenclature would have it, the Rat lungworm really likes to live in a rat’s lungs. But the rodents keep coughing them up and out, sending them slithering off to hitch a ride on slugs and snails until they can insinuate their way into another rat’s breathing passages. Our unwell med student and several of his fellows ingested the unhappy creature when they all shared a bit of salad during a field trip to Jamaica. Apparently, nobody washed the slug off the lettuce.

“Monsters Inside Me” will probably make viewers think twice about eating uncooked meals in exotic locales. It should also do wonders for the sale of salad spinners.

As any viewer of doctor shows surely knows, our med student was not being sickened by the Rat lungworms, but the human antibodies attacking the invader. After a heavy dose of steroids suppress his immune system and stop the brain swelling, his unwelcome visitors are allowed to die on their own.

The med student survived to raise a family and eat well-washed lettuce. He won’t let his bout with meningitis keep him down, though. “I can’t,” he quips. “Otherwise, the worms win.”

• “American Masters” (7 p.m., PBS, check local listings) profiles prominent National Public Radio’s famous monologuist and novelist on “Garrison Keillor: The Man on the Radio in the Red Shoes.”

“Keillor” follows its subject over the course of a year as he hits the lecture circuit and mingles with fans of his long-running “Prairie Home Companion” and “Lake Wobegon” books, both fictional evocations of small town life that allow him to offer his wry and well-worn commentary on the passing scene.

Tonight’s other highlights

• “America’s Got Talent” (7 p.m., NBC) continues for two hours.

• “So You Think You Can Dance” (7 p.m., Fox) continues.

• A police chase proves chillingly effective on “CSI: NY” (9 p.m., CBS).