In homes with hand sanitizers, the risk of catching a gastrointestinal illness from a sick child dropped 60 percent compared with the control families. The two groups did not differ in rates of respiratory illness rates, but families with the highest rates of sanitizer use had a 20 percent lower risk of catching such an illness from a sick child.

Regular soap and water and alcohol-based hand sanitizers are both effective in eliminating the H1N1 virus from the hands. In February, researchers in Australia coated the hands of 20 volunteers with copious amounts of a seasonal H1N1 flu virus. The concentration of virus was equivalent to the amount that would occur when an infected person used a hand to wipe a runny nose.

When the subjects did not wash their hands, large amounts of live virus remained even after an hour, said the lead author, Dr. M. Lindsay Grayson, a professor of medicine at the University of Melbourne. But using soap and water or a sanitizer virtually eliminated the presence of the virus.

Frequent hand washing will not eliminate risk. When an infected person coughs or sneezes, a bystander might be splattered by large droplets or may inhale airborne particles. In a recent Harvard study of hand sanitizer use in schools, hand hygiene practices lowered risk for gastrointestinal illness but not upper respiratory infections.

Still, it is a good idea to wash your hands regularly even if you’re not in contact people who are obviously ill. In a troubling finding, a recent study of 404 British commuters found that 28 percent had fecal bacteria on their hands. In one city, 57 percent of the men sampled had contaminated hands, according to the study, which was published this month in the journal Epidemiology and Infection.

“We were surprised by the high level of contamination,” said Gaby Judah, a researcher at the London School of Hygiene and Tropical Medicine. Ms. Judah added that many of the contaminated commuters reported that they had washed their hands that morning. They may have been embarrassed to admit they hadn’t washed, or they may have picked up the bacteria on their hands during their commute.

For all those reasons, the Centers for Disease Control and Prevention, with other health organizations around the world, urge frequent hand washing with soap and water or alcohol-based hand sanitizers. (They also repeat some advice you may not have heard from your mother: cough or sneeze into the crook of your elbow, not your bare hands.)