When are you contagious?

You are contagious for the first 5 days from the moment you start feeling sick or had a fever.  So stay home, or don't touch anything and cough into your elblow. 

1. How long are you contagious when you have a cold? - The Boston Globe
   ...One study showed that when one spouse is experimentally infected with a virus, the other spouse typically gets infected within the first three days.
   ... the virus starts reproducing within half an hour of entering the nose; it takes only 8 to 12 hours for new virus to appear in nasal mucus.
2. WikiAnswers - When are you most contagious when you have a cold
3. Go Ask Alice!: When are colds contagious?