Influenza, or the flu, attacks up to one billion people annually. In the US, it kills 20,000 annually, most of whom are children or elderly. Occasionally the flu becomes pandemic: in 1918, it killed 20 million people worldwide. The flu is a very contagious viral infection spread through the air by coughing, sneezing, or simply talking. It is not caused by getting caught in a rainstorm or by sleeping with the fan or air-conditioning on.
The incubation period is about three days. It doesn't sneak up on you, like a cold does. All of a sudden, you feel weak, you have a high fever, you have chills, you cough frequently and forcefully, your throat is sore, and your body aches.
For most adults, the treatment is to simply wait it out: stay home, get lots of bed rest, drink lots of fluids, and take over-the-counter medications such as aspirin, pain-killers, and nasal decongestants. Symptoms usually go away within two weeks. For the elderly and young, the initial viral infection may become a bacterial infection with deadly consequences, because the victim becomes too weak to battle the disease. The death rate for the general population is about one in 1,000. Those most susceptible to severe effects of the flu are people over 65 and people with chronic heart or lung problems, such as asthma.
Flu season in the US is usually December to March. The best prevention, of course, is to stay away from infected people. Since that is almost impossible, the next best preventive strategy is to get an annual flu shot. This vaccine reduces the number of people who get infected, and who die, yearly.