Q. Recently, a friend — with a hoarse voice, a cough and a stuffy nose — told me, “Don’t worry. I’m not contagious.” Is she right?

A. It may depend on how long she has been sick. If your friend has a common cold, she will be infectious for about three days after the onset of symptoms. If she has the flu, she will be infectious for about a week.

The best data we have on the infectivity of upper respiratory infections comes from volunteer studies, in which healthy individuals volunteer to be infected with viruses like influenza, the cause of flu, or rhinovirus, the most frequent cause of the common cold. Understandably, the number of such studies is small.

Those studies show that symptoms are an undependable marker of infectivity. A more reliable guide is the natural course of infection, which can be divided into three phases: incubation (infectious without symptoms), symptomatic/infectious, and recovery (noninfectious with symptoms).