It's not hypocrisy, exactly, but many very good, very sane parents are capable of talking out of both sides of their mouths when the subject is the common cold. Just ask them: Should a child be kept home from day care, school, playdates, tumbling classes, and the like when she has a cold?

Of course! Because who wants a snot-nosed, sneezing kid spreading germs on another child—especially when that child is yours?

But ask the same question on a morning when parents have to get to work and their own 4-year-old has just started to sniffle. Now they're more likely to think going to school is a good idea. There's no one who can stay with the child at home, is there? And besides, this might not really be a cold. . . .

It probably is a cold, though—at its most contagious. Symptoms like a runny nose, sneezing, coughing, and tiredness are caused by the body's immune response to the virus; by the time they show up, the virus has been in there for two or three days and can readily be spread to others for the next three to four days. "Children are most contagious early on, sometimes before they even have symptoms," says Conrad J. Clemens, M.D., an associate professor of pediatrics at the University of Arizona College of Medicine, in Tucson.

That's just one reason it is so difficult—impossible, really—to protect your own child from being infected by other kids. Another is that kids are such efficient transmitters of viruses. They touch one another and one another's toys so frequently, it's like building an expressway for cold germs. "Little kids explore the world by putting things in their mouths," says Lisa Chamberlain, M.D., a pediatrician in Palo Alto, California, who teaches at the Stanford School of Medicine, "and they don't give each other personal space, so they're just going to get [bugs] more often than we are."

Next Page: Up to age 1, babies can have a harder time dealing with colds than older children do.