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The flu (influenza) is a highly contagious disease that can infect anyone. The virus is primarily transmitted in coughs and sneezes, but can also occur on contaminated objects or surfaces. It is transmitted most rapidly among dense populations. Its seasonal occurrence in winter is largely related to people being inside and exposed to other people more during cold weather.

People with weakened immune systems have an increased risk of contracting a flu infection, as well as developing severe symptoms or complications of the flu. Many different factors can contribute to a weakened immune system, including certain medical conditions, certain medications as well as lifestyle factors such as cigarette smoking and drinking large amounts of alcohol. Children are especially likely to contract a flu infection because of their immature (still developing) immune systems.

While anyone may be infected with the flu, certain populations are at greatest risk of severe symptoms or complications of the flu. They include:

• Adults 65 years of age or older
• Children younger than 24 months
• Pregnant women
• Patients with certain chronic medical conditions including malnutrition and lung disease