'The flu' has become a slang term that refers to almost any ailment where you just don't feel quite right.
But influenza proper is a handful of specific viruses that invade the respiratory lining, evade the immune system, and mutate at a rate that even the world's most brilliant doctors, scientists, and health organizations struggle to keep up with.
So what is Influenza exactly, and why does it always seem to be one step ahead of us?
Some of the confusion around the term 'flu' may stem back to the word's roots:"Influenza" is an Italian word that just means, 'influence'.
Because, back 500 years ago, people didn't know about germs then.
They thought if you got this disease, you were under the influence, perhaps of the stars or something else.
My name is David Morens.
I'm a medical doctor and an epidemiologist, and a virologist, and a historian, at the National Institutes of Health, and an officer in the United States Public Health Service, and I'm here today to talk about one of my favorite subjects: influenza.
Influenza presents with such iconic symptoms that many other diseases are classified by their similarity to it.
Tell your doctor you're experiencing 'flu-like symptoms', and they'll know exactly what you mean: coughing, fever, head and muscle aches, fatigue, and well, just general "malaise", which kind of means just feeling really bad.
