Influenza, commonly known as "the flu," is a highly contagious viral infection of the respiratory tract. It affects all age groups, though kids tend to get it more often than adults.

In the United States, flu season runs from October to May, with most cases occurring between late December and early March.

The flu is often confused with the common cold, but flu symptoms usually are more severe than the typical sneezing and stuffiness of a cold.

Symptoms, which usually begin about 2 days after exposure to the virus, can include:

Infants with the flu also may seem fussy all of a sudden or just "not look right."

After 5 days, fever and other symptoms have usually disappeared, but a cough and weakness may continue.

All symptoms are usually gone within a week or two. However, it's important to treat the flu seriously because it can lead to pneumonia and other life-threatening complications, particularly in infants, senior citizens, and people with long-term health problems.

The flu is contagious, spread by virus-infected droplets that are coughed or sneezed into the air. People who are infected are contagious from a day before they feel sick until their symptoms have ended (about 1 week for adults, but this can be longer for young kids).

The flu usually occurs in small outbreaks, but epidemics — when the illness spreads rapidly and affects many people in an area at the same time — tend to occur every few years. Epidemics often peak within 2 or 3 weeks after the first cases occur.

When an epidemic spreads worldwide, it's called a pandemic. The most recent flu pandemic occurred in 2009, with the H1N1 ("swine") flu.