Symptoms: fever, aching muscles, chills and sweating, headache, coughing, congestion, and sore throat.
The flu affects the respiratory system because of the congestion, sore throat, and coughing.
Most of the time the flu is not lethal, unless it is bad enough. It is not chronic, but can be acute.
To prevent the flu you can get a flu shot. But if you already have the flu, there isn't really a way to treat it. Just make sure the rest and drink lots of water because most of the time you become dehydrated. It should only last about a week.
Millions of people get the flu, just in the US. Because the strain evolves quickly, it is able to spread around quickly too.
Works Cited: http://www.mayoclinic.org/diseases-conditions/flu/symptoms-causes/dxc-20248062