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We ask the experts to settle common questions we've all wondered about.

QUESTION

If I've had a cold for a week, am I still contagious?

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ANSWER

First, you should make sure you have a cold and not something more severe like influenza - commonly known as the flu. They may be hard to tell apart, but with a cold, you usually get a sore throat, nasal congestion or runny nose, sneezing and coughing, with or without a fever. With influenza, which can be life-threatening if you are older or have a weakened immunity, typical symptoms are fever, cough, tremendous fatigue, and muscle aches and pains.

Although you can prevent influenza to some degree with a vaccine, there is no long-lasting immunity to the common cold. There are many viruses that cause the common cold, such as rhinovirus, coronavirus and adenovirus to name a few. This may account for the differences from one cold to another, and may also explain why there is no lasting immunity.

On average, an adult may get one to three colds a year. Children tend to catch more colds. A healthy child gets three to six of them a year.

The typical cold lasts four to seven days. When a contagious individual coughs or sneezes, respiratory droplets containing infectious viral particles can spread to others. You "catch" the cold from close exposure to these droplets (usually within one metre of a cough or sneeze), or by touching a surface with these virus droplets on it, then directly transmitting the virus from your hands to your nose or eyes.

Generally, you are most contagious, or at the highest level of transmitting the cold virus, during the first one to three days of your cold or of having symptoms. Some transmission risk still remains on the fourth and fifth days, and by the sixth and seventh days there is virtually no risk.

If you are otherwise healthy and catch a cold, the most important thing to remember is to avoid visiting those family members and friends who are in hospitals, nursing homes or living with other immune-compromised, vulnerable individuals. A simple cold may further weaken their systems, allowing for the development of a more serious infection.

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Dr. Andrew Simor is chief of Microbiology and Infectious Diseases at Sunnybrook Health Sciences Centre. A University of Toronto professor, Dr. Simor is co-chair of the Canadian Nosocomial Infection Surveillance Program.

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