The problem with this question is that you are contagious after certain period of exposure, but may not have any idea that you've been exposed because you may be asymptomatic with things like influenza. Hypothetically after the first 24 to 48 hours you would be at risk of contagion, but may not show symptoms until later. Also depending upon the strain type, the amount of time you are sick and the time you will recover can vary greatly thus making the contagion time different for different strains of the flu.