These two groups cause high fever, muscle aches and pains, lower back pain, cough, and runny nose. The flu can be so bad you just can't get out of bed. Sometimes these viruses can get deep in the lungs and cause pneumonia. When the Influenza virus gets in the lungs it damages the cells that line the inside surface of the lungs. If the right bacterium gets in these damaged lungs they can cause secondary bacterial pneumonia.
Pneumonia can be a life threatening situation.
Many people die each year due to pneumonia from the Influenza virus or to secondary bacterial pneumonia. A yearly flu shot is required because the Influenza viruses change so frequently. The common Influenza viruses of last year will not be the common ones we see this year. Therefore every year vaccine manufacturers must make a new vaccine. Influenza viruses can be quite different from each other.