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From Cecil's Mailbag by the Straight Dope Science Advisory Board

How long are colds contagious?

Dear Straight Dope:

Is it true that after a few days a common cold is no longer contagious? --Mitch Rubenstein, Chicago, IL

SDSTAFF Jill replies:

The "common cold," acute viral rhinitis, is actually caused by more than a hundred different agents. (The same symptoms can also be caused by coronaviruses, influenza virus, adenoviruses, and respiratory syncytial viruses [RSV].) That's why we get colds over and over again, and it's also why immunity against some of these bugs can be short-lived. About half the time, the cause of a common cold cannot be identified.

The mode of transmission is presumed to be by direct contact or by inhalation of airborne droplets, or indirectly (and more importantly) by hands and coming in contact with anything contaminated by snot or spit from an infected person. Shake hands or use the telephone right after that sneezing secretary, then touch your eyes, and you've got it, babe. Depending on the specific germ, the incubation period is between 12 hours and 5 days, usually 48 hours.

To answer your question, the common cold is "contagious" between 24 hours before onset of symptoms until 5 days after onset. Like everything else this varies depending on the organism, but that's a pretty safe estimate. This period of communicability was determined by taking nasal washings of experimentally infected volunteers. There is also the possibility of healthy carriers, though this appears to be rare with rhinoviruses.

How can we prevent contracting and transmitting colds? Stay home from work if you're sick, dammit. I'll write you a note for your boss. Wash your hands frequently, of course. Cover your mouth when you cough or sneeze, but DO NOT USE YOUR HAND for the above reasons. It's good to get into the habit (and teach your kids, too) to cough and sneeze into the crook of your elbow instead. There is also a vaccine specifically against flu which you should get in October or November each year (the particular flu bug varies each year, so it's a yearly vaccination). That won't prevent you from catching colds, though. Try to stay out of jail, off ships, out of the military, and away from any other place you'll be stuck in close quarters with a crowd of people who cough, sneeze, and leave snot rags around.

Most of the above information is from The Control of Communicable Diseases Manual, Abram S. Benenson, Editor, sixteenth edition (1995). This is a really fun little compendium of infectious bugs from hantavirus to Ebola to venereal warts. If you love communicable diseases as much as I do, this book is a Must Have. You can order it from: The American Public Health Association, 1015 Fifteenth Street NW, Washington, DC, 20005, or check your local medical school bookstore.

--SDSTAFF Jill
Straight Dope Science Advisory Board