Even casual fans of the vampire genre know that an undead bloodsucker can enter a room only after he’s been invited. At the very least, you have to admire his manners. And aficionados of animal programming know that the obnoxious pets rescued and re-educated on shows like “Dog Whisperer” and “My Cat From Hell” are often considered members of the family.

So it’s a little terrifying that some of the most dangerous creatures of all never wait for an invitation — and that some have been living with us for years. Now in its third gut-wrenching season, “Monsters Inside Me” (7 p.m., Animal Planet) explores those tiny microscopic bugs that dwell inside every human being. Most of the time, such bacteria help us digest food and keep our inner life in balance. But, to paraphrase the late disco queen Donna Summer, when they’re bad, they’re oh so bad!

The first installment in this queasy season sports a suitably tabloid title: “My Child Will Only Eat Cat Food.” Not to give too much away, but the Friskies-loving tyke may be suffering from a deadly elevation in white blood cells arising from the activities of a deadly roundworm. And this child doesn’t have nine lives.

Week after week, “Monsters” exposes deadly parasites that live inside people, causing extreme pain and devouring flesh inside the body and on the skin’s surface, which drives them to bizarre behavior and takes at least one subject to the brink of suicide.

Bon appetit!

• The 2007 comedy “Walk Hard: The Dewey Cox Story” (8 p.m., Fox Movie Channel) lampoons the pieties of big-screen musical biographies and skewers a virtual pantheon of rock legends. John C. Reilly acquits himself well as a singer.

Tonight’s other highlights

• The new series “John Fugelsang: So That Happened” (5 p.m. and 8 p.m., Current) reviews the past week in popular culture.

• A serial arsonist strikes again on “CSI: NY” (7 p.m., CBS).

• On two episodes of “Grimm” (NBC), a church robbery (7 p.m.), unhappy tidings (8 p.m.).

• Major League Baseball Wild Card Game (5 p.m., and 8:30 p.m., time approximate, TBS).

• A letter from prison proves illuminating on “Made in Jersey” (8 p.m., CBS).

• The team returns to the Harvard lab on “Fringe” (8 p.m., Fox).

• A case of domestic violence is anything but routine on “Blue Bloods” (9 p.m., CBS).

• A budget meltdown forces Kane to compromise with his enemies on “Boss” (8 p.m., Starz).