The influenza virus causes more severe symptoms and can cause more severe complications than other respiratory viruses, such as cold viruses. The elderly, people whose immune systems are impaired, and people who have chronic medical problems are at risk for more severe flu symptoms or complications. A national epidemic of flu is common every year between October and April. The influenza virus can infect you when you are exposed to an infected person's upper respiratory fluids. For example, droplets released into the air by coughing or sneezing can carry the virus.
Influenza tends to start suddenly.
You may feel fine one hour and have a high fever the next. These acute symptoms usually last 3 to 5 days. They often start improving gradually after the first 48 hours or so. Infection with the flu virus often leads to other infections. Ear, sinus, and bronchial infections are examples of such infections.