Other viruses usually cause a Flu-like illness that happens in the summer. The viruses that cause Flu live in the nose and throat and are sprayed into the air when an infected person sneezes, coughs or talks. People nearby can then inhale the virus. Flu symptoms usually start one to three days after a person inhales the virus and up to a week later. The Flu can be very dangerous.
Most people are only sick for a few days.
However, the flu can be serious in the elderly and in people with chronic medical problems or weakened immune systems. Some people may develop dangerous complications such as pneumonia. The Flu shot, like any other vaccine, can cause serious side effects in some people. The most common side effect is a sore arm, which lasts a day or two. A few people, mostly children, have a mild fever, headache, chills or muscle aches for two days.