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The question

How do you know for sure whether it's the cold or a flu? I was home sick two days this week, and even though I had my flu shot last week, I felt pretty darn fluey. Is there a key symptom to distinguish the two?

The answer

The cold and the flu are both respiratory illnesses caused by viruses, but while they can share similar symptoms, the flu is much more severe than a cold.

Colds usually have a gradual onset over one or two days with symptoms such as runny nose, sore throat, nasal congestion and mild fever. Symptoms generally last a week to 10 days although a residual cough or runny nose can last for a few weeks.

To compare, flu symptoms can come on quickly within a few hours, knocking you off your feet due to the key symptoms of fatigue, weakness, aches, headache and high fever (>38.5 degrees Celsius). While the flu generally lasts just a few days, the fatigue and exhaustion can last a few weeks after the initial infection has resolved.

Although you received the flu shot last, it can take up to 2 weeks after immunization to develop protection against the flu so it is possible that you may have indeed had the flu. While the flu shot is one of our best defenses against the flu - you can still get the flu but generally it is much milder than if you had not had the immunization.

Regardless of whether you have the cold or the flu, treatment for both is based on managing symptoms, rest and hydration. For those at higher risk of complications from flu such as the elderly, young children or those with compromised immune systems, antiviral medication can be prescribed. Antibiotics do not play a role in treatment of either the cold or the flu as they will generally run their course with good rest, hydration, and symptom relief.

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The content provided in The Globe and Mail's Ask a Health Expert centre is for information purposes only and is neither intended to be relied upon nor to be a substitute for professional medical advice, diagnosis or treatment.