Just a patch or a plague?

It can propel entire families to flee their homes to bunk in cramped hotel rooms, leaving cherished possessions behind to rot. It can make a condo resident accuse a neighbor of being so dirty that property values have fallen, and a landlord worry that it will be impossible to find a new tenant.

"The black plague." "Leprosy." "The new asbestos." The terms for toxic mold sum up the fear and stigma attached to it. One Bel-Air woman was so afraid of what people would think that she asked the company hired to clean up the mold in her house to cover that word on the side of its trucks.

Contaminated homes have been cordoned off as if they were crime scenes, bulldozed and burned.

Celebrity cases have moved mold into headlines. Jennifer Aniston found it growing in the garage of the $15-million Hollywood Hills home she shares with husband Brad Pitt; she flew to Malta to join Pitt while a crew came to the rescue. Toxic-crusader Erin Brockovich got sick from it; she is suing the seller and has settled with the builder. Ed McMahon fell ill, and his dog died; he got a $7-million insurance check.

How dangerous is this stuff? Does a fuzzy splotch on the shower curtain mean it's time to call in the mold removers in moon suits?

No.

"Having mold in a bathroom is not unusual," says Dr. Eckardt Johanning, director of the Fungal Research Group Foundation in Albany, N.Y. "Mold grows where you have water, moisture or high humidity. If you see it, just don't wait until it's several square feet wide before you do anything about it."

Before springing for air-quality test kits with petri dishes or calling in the mold-sniffing dogs, take a crash course on the subject. What you'll discover is calming. Or maybe not. For you see, mold is natural. And it's everywhere.

Mold is a fungus, a family that also has some good relatives like mushrooms and yeasts. To grow, it has to have something to drink (the tiniest drop of water), eat (anything soft) and a little warmth. There may be hundreds of thousands of types; no one is sure. Some make penicillin and ripen chevre cheese, while others can be harmful. "The bad boy is Stachybotrys, the nasty, black toxic mold," said Chris Vuckovich of Aaaaable, an Orange-based company that specializes in mold and water damage. "By the time you see that, you've got a problem."