Tierno's research indicates that your carpet probably contains about 200,000 bacteria per square inch, making it 4,000 times dirtier than your toilet seat. "Rugs are botanical and zoological parks," says Tierno, who says hundreds of thousands of different types of species live there. These invasions occur because the average person sheds about 1.5 million skin cells every hour; these skin cells hit the rug and serve as food for germs. Add in food particles, pollen, and pet dander, and you have a gratis buffet, he says. And since a vacuum cleaner's suction and rotating beater brush don't usually reach the bottom of the carpet, you're bound to have communities of E. coli, salmonella, staphylococcus, and other bacteria down there. Every time you walk on the carpet or roll around on it with your kids, you disrupt the bacteria, bringing some closer to the surface, Gerba says.

Your cleanup: Hire a company to do a deep steam-cleaning at least once a year, and consider covering high-traffic areas with machine-washable area rugs.

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