Influenza is something most people get at least once and most of the time multiple times in their lifetime. It can lead to pneumonia especially in the elderly, malnourished, and people with chronic lung or heart problems.
Normally every year, about 25 to 50 million Americans catch the flu and between 10,000 to 40,000 die from it.
The virus has tons of genetic material protected by a sphere protected by layers of fat and protein. All this genetic material is like having hundreds of pieces of legos which you can make tons of different shapes with. Thus, it mutates easily and frequently. This means you'll probably never be immune from the flu.
Influenza epidemics are probably as old as human history. It is hard to trace it's path because of it's genetic symptoms.
It probably plagued Athens in 430 B.C. and destroyed Charlemagne's army in 876 A.D. There is also strong evidence of an outbreak in the 16th century too. The first recorded pandemic was in 1580 across Africa and Europe. It killed thousands in 1647 as it moved from the Caribbean to New England.
It is often known by colloquial names such as "la grippe," "jolly rant," or "the new acquaintance." It was after the epidemic of 1732-33 in the American colonies that an English doctor named John Huxham introduced an old Italian folk term which connected the colds, cough, fevers to the astrological "influence" of the stars. Thus came the popularity of the term influenza. However, it never really attracted much attention in history as other diseases stole the show – that is until the pandemic of the "Spanish flue" in 1918-19.
This pandemic is 20th century's worst and deadliest faced by any modern western society. Contrary to the name, the epidemic seemed to have started in the U.S. By conservative estimates, 21 million people died worldwide out of a billion infected. At least 12 million died in India. There were about 550,000 deaths in the U.S. 1/10th of American workers were sick in bed during the winter of 1918. 1/5th of the U.S. Army and 28% of the civilians caught the flue. About 20,000 New Yorkers died fall of that year. Inuit villages in Alaska were wiped out. Samoa lost 20% of its inhabitants. However, no blood samples were saved from this epidemic so it will never be known exactly what type of virus caused this epidemic. Epidemics like this could happen again. Influenza viruses such as the Hong Kong flu, which killed about 70,000 people in the U.S. in 1968-9, remind us that influenza should not be completely ignored.