When are colds and the flu most contagious?

Colds. Stress, poor diet, diabetes and other things may impair your immune system as well as contact with kids and frequent flying.
When symptomatic. Virus is shed and transmitted in the respiratory secretions of the symptomatic patient (sneezing, fever, chilling, coughing).

Related Questions

Are you stil contagious after 24 hours of having the flu or cold?

Yes. After having 24 hours of cold or flu symptoms, a person is still quite contagious. It is hard to tell when a person is not contagious, because that varies from case to case, and because the viruses decrease over a few days (they don't all die at exactly one moment in time). The safer thing to do is to avoid spreading viruses by touching others, or by sneezing or coughing on others, for a week. Read more...