The flu is a respiratory (i.e., nose, throat, and lung) infection that can be caused by a variety of influenza viruses. Many people use the word "flu" when they actually have a cold. Although the common cold is also caused by viruses, the flu and common cold differ in several ways.
In North America, the flu almost always strikes between November and April. Up to 25% of the population may be infected in an average year. Stronger epidemics (i.e., when the flu occurs in more people than expected in a given area or season) come every 2 or 3 years, infecting twice as many people as during an "off" year.
Most people who get the flu will recover within 1 to 2 weeks, but some people are at risk of developing complications such as pneumonia. On average, about 4,000 to 8,000 people in Canada die each year from complications of influenza, and about 20,000 people with the flu are hospitalized. Most of these people have other medical conditions, are seniors, or are very young children.