The common cold is actually a catchall term for infections caused by at least five major varieties of viruses. These range from rhinovirus, a cousin of the polio virus, to influenza, which in its more virulent manifestations can cause a deadly pneumonia. The 1918-19 influenza epidemic killed 15 million to 25 million people worldwide.

Both colds and flu can produce active infections without showing any obvious symptoms, a fact that experts don't yet understand.

On the average, adults catch two to four colds annually, children catch six to eight colds, and nursery school-age youngsters catch even more. One 10-year study of families found that children in day-care centers averaged one cold a month during the school year.

Boys get slightly more colds than girls. But the tables turn in adolescence and stay that way through most of adulthood. Researchers believe this pattern may reflect the normally closer contact women have with young children.

Colds are spread more often via the home and the classroom. Children usually catch the virus at school, bring it home and infect the rest of the family. Studies show it takes two to five days for a cold to spread within a family. As a result, parents contract more colds than adults without children.

Cigarette smokers average the same number of colds as nonsmokers. But smokers are likely to suffer more severe symptoms and their colds are apt to linger up to twice as long as nonsmokers'.

Tonsillectomies offer no protection against catching colds.

Cold sufferers appear to be the most infectious on the third day of a cold, when virus shedding peaks.

Despite the saying that catching a chill can produce a cold, researchers report that "the effect of cold weather per se . . . does not appear to be the explanation for the seasonal outbreaks of colds." There is some evidence, however, that as a cold develops, the body has more trouble regulating temperature, thus the cold sufferer may remember feeling chilled and think that caused the cold.