usually antibiotics start working within 24-48 hours. so in most circumstances a person should not be contagious after the first three days. if it is a viral infection though antibiotics will not treat that. so you can still be infectious especially if you are coughing or sneezing. talk to your doctor if you are not feeling better or are feeling worse.
on the 4th day of a 10day zpack am i still contagious?