Most experts agree that adults with a cold or the flu start being contagious about a day before they start experiencing symptoms. For the flu, the contagious period then lasts five to seven days into the illness. For children, the contagious period for the flu can last up to two weeks after they start feeling sick, even if they start feeling better before that. The contagious period for a cold lasts about three to four days into the illness. As a general rule, people with a cold are most contagious about three days after their initial exposure to the virus. For more information on colds, influenza and related topics, check out the links on the next page. Print.