Nothing warms up a room or feels better under bare feet than thick lush carpeting. Nothing provides a more inviting environment for all sorts of germs, either.

Even carpeting that’s relatively new can host enough bacteria, viruses and other nasties to set off allergy attacks or cause serious illness, according to research from two noted microbiologists.

Both Chuck Gerba, PhD, of the University of Arizona, Tucson, and Philip M. Tierno, Jr., PhD, a microbiologist at NYU School of Medicine in New York, assessed home surfaces to find out which ones were germiest. It turns out that home carpeting can contain about 200,000 bacteria per square inch. In comparison, Gerba found that a typical toilet seat has a mere 49 bacteria per square inch.

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What’s down there?

One of the microbes Gerba’s germ detectives found in carpet was fecal bacteria, especially in homes with dogs or kids in diapers. So do noroviruses, the bugs that can cause much GI distress and have infected many a cruise ship.

"Norovirus has been transmitted by carpet," says Gerba, citing outbreaks in hotels in which investigators from the Centers for Disease Control found that carpeting tested positive for the virus.

Some of the stuff that winds up in carpet falls from the air or travels in on the bottoms of shoes, which carry many organisms and debris. When Gerba looked at shoes that had been worn at least three months, he found up to 8 million bacteria per square centimeter. (You can minimize dirt and debris on carpet by wiping feet vigorously on the doormat before entering the house or, even better, taking off your shoes and stowing them by the door.)

Then there’s allergens. Dust mites thrive in carpets, Gerba says. Other allergens that can lurk in carpets include dust mites, mold, animal dander and saliva, urine from pets like rabbits, hamsters and mice.

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Carpet can also harbor a witch’s brew of human skin cells and hair, insect parts, sweat, saliva, mucous, fungi and fungal spores food particles, pollen, cosmetics, soil and sand, says Tierno.

Keeping it clean

Vacuuming isn’t perfect — sometimes it can make things worse by moving bacteria and viruses around, says Gerba — but you should do it regularly. The Carpet and Rug Institute (CRI), an industry group, recommends vacuuming high-traffic areas every day and cleaning up spills and pet accidents immediately. Tierno advises using a vacuum with a high-efficiency particulate air (HEPA) filter at least weekly. If you top your carpet with area rugs, they should be washed and dried every week, according to the AAAAI.

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The CRI recommends having your carpets professionally deep cleaned every 12 to 18 months to remove embedded dirt and grime. Deep cleaning is sometimes called hot water extraction or steam cleaning.

Next time you're in the market for a new vacuum, consider one with an ultraviolet (UV) light feature. Gerba studied UV light and found that it can reduce the number of bacteria in carpeting by about 90 percent. Other research has found that vacuums that include UV disinfection may be capable of zapping twice as many germs as regular vacuuming

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Kathleen Doheny is a Los Angeles journalist specializing in health, behavior and fitness topics.