You are most contagious right about the time you feel symptoms starting. But you should assume you’re able to spread the virus the entire time the cold lasts - typically 3 and 10 days. The contagious period for the flu is similar. A study found you’re most contagious during a 2-3 day period after the on-set of symptoms. But just like a cold, you can still spread influenza during the entire time you’re sick - anywhere between 2 and 10 days.