Am I Contagious?

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 such as 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).

The incubation phase lasts about a day for influenza and may be just a few hours for the common cold. During this time, someone can spread infection, but there are no telltale symptoms so there is little we can do to protect ourselves. We shake hands, spend time together in enclosed spaces and do all manner of things that unwittingly increase the risk for disease transmission.

During the symptomatic/infectious phase, symptoms like coughing and sneezing serve as a warning sign to avoid direct contact with a sick person, and especially to avoid exposure to respiratory secretions. The symptomatic/infectious phase lasts about five to seven days for flu and about three days for the common cold. Beware, however, that some people, including those who have been vaccinated against the flu, may have only very mild symptoms but can still be contagious.

The recovery phase begins within about a week. During this time patients may still have symptoms such as cough or fatigue but are no longer infectious to others.

So if your friend says, “I may sound bad, but I feel fine. I know I’m not contagious,” don’t take her word for it.