For how long is a cold contagious?
The primary period of contagiousness starts during the two-to-three day window before symptoms surface and lasts throughout the first five-to-seven days of a cold. Colds tend to be most contagious in the early part of that window, right during the onset of symptoms.
Colds are transmitted through what we call respiratory droplets, so if somebody has a head cold, they're more likely to be sneezing and blowing their nose a lot, which is how they would transmit the cold to other people. But if someone has a chest cold, they’re more likely to be coughing and transmitting the virus that way.
Research (and experience) shows that young children on average get more colds than adults (around 8 per year vs 2-3 for adults). Besides having less mature immune systems, they tend to share germs more readily than adults. This doesn’t mean that they are not necessarily more contagious, but they are not as mindful about general hygiene, which results in easier transmission of their respiratory secretions to others.
When it comes to deciding when to return to work or resume your regular routine when you have a cold, the answer is going to be a little different for everybody. The two main things to consider are recuperation and transmission. In terms of recuperation, you want to restrict your physical activity enough for you to give your body adequate rest. Taking some down time will let your system use more of its resources toward battling the infection. On the flip side, there has been some research done on people who maintain their exercise habits while sick, and in some cases, exercise has been shown to actually help resolve infections more quickly.
In short, the answer varies from person to person. If you feel like you've got the energy to exercise while you have a cold, then there is really no harm in doing so and the activity might even be a benefit to your immune system. Otherwise, rest will help you fight the infection.
It gets a little harder to give concrete advice when it comes to transmission because this depends on the nature of your work. For example, if you're a cashier and interacting with a large number of people is part of your work routine, you may want to wait longer before returning to work to limit spreading your germs. But if your job environment is more isolated, then you can use your best judgment in regard to getting back in the swing of things.