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Is Soap "Self-Cleaning"?An answer to the Explainer's Question of the Year.

By Daniel EngberPosted Wednesday, Jan. 3, 2007, at 7:11 PM ET

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Arthur Allen reported on Europe's war against hospital infections. Daniel Engber explained how European peat bogs keep things fresh for centuries and why washing your fruits and vegetables won't help if they're covered with E. coli. Eliza Truitt rated the various cleaners you can use in a washing machine.

Still, washing with soap and water has been repeatedly shown to prevent the spread of illness, and may be helpful even when it increases your bacteria counts. That may be because two kinds of microbes live on the hands: residents and transients. (In fact, they can even protect your skin from more malicious microbes.) The transient variety are the ones that tend to cause colds or other infections—the ones you want to get rid of when you wash your hands. It's possible that the increase in bacteria that can result from a hand-washing is composed of harmless residents, not dangerous transients.

According to the guidelines from the Centers for Disease Control and Prevention, hand-washing remains a very important method of staving off infectious disease, and either bar soap or liquid soap should be used after a trip to the bathroom or before a meal. Local health agencies and inspectors are sometimes more wary of bar soap. They either ban it outright or suggest that the bar be placed on a draining rack to dry out between washings. (The gooey bars are more likely to harbor germs.)

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Explainer thanks his brother Benjamin Engber, Elaine Larson of Columbia University, and Sam Sober of the University of California.