Staying home sick when you're actually sick is no fun for anyone, but knowledge that you are still a hulking heap of virulent plague is arguably worse. Wondering if you are or aren't contagious can throw off your relationships, screw with your best-laid plans, and turn your illness into a table tennis match if you happen to live with a roommate.
Luckily, science has mostly figured out how to cure a cold, as well as how long you need to worry about it after you've gotten rid of it.
How long is a cold contagious?
Most experts agree that a cold is contagious before you even start showing symptoms, and can increase in virulence with the severity of symptoms. It usually takes 24 to 72 hours for your cold to develop into a flaring-nostril, rivers-of-snot levels of bad. A common cold is most contagious in the first two to three days of infection, and typically not contagious after a week. By then your symptoms have probably subsided.
How soon can anyone really say, "Don't worry, I'm not contagious"?
Just don't ever say this if you're still sick. On the most basic level, there's little way for you to intuit how contagious you actually are until you're symptom-less and sufficient time has passed. It's one of the most basic of common cold myths, right up there with "I caught it from my girlfriend."
The fact is there are innumerable ways to catch strains of the common cold that could have nothing to do with your loved ones, and you don't magically know whether you're contagious or not based on your feelings until about three days after you stop seeing symptoms.
How to avoid catching someone else's cold...
These may sound obvious, but hey, people are still catching colds and we've been walking upright for 1.9 million years or so. Clearly it hasn't sunk in yet. The CDC offers these tips to avoid catching the cold:
Wash your hands. With soap and water (or alcohol-based sanitizer if not available), in between your fingers, paying close attention to the tips, for at least 20 seconds. Channel the god Judi Dench as Lady Macbeth if you have to.
Refrain from touching yourself. You should avoid touching your eyes, nose, and mouth if your hands aren't washed. (If they are, wash them again.)
Avoid people. Colds may strike in the winter, but there's little evidence that cold weather itself contributes to their spread. What's more likely is that people huddle up in large indoor groups in the winter and all our collective flesh piles trade the viruses like Pokemon cards. Don't hang out with folks who have colds.
Whatever, people suck anyway. Pour your sickly self a hot toddy and enjoy the night in.