Everyone knows that influenza refers to the flu, but not many are aware of its dark history and the millions of lives it has left in its wake.

The symptoms of the flu are easy to recognize, such as fever, sore throat, muscle pains, severe headache, coughing, weakness and fatigue. It usually affects the upper airways and lungs.

The first recorded medical description of a disease with flu-like symptoms was in 412 BC when no less than the great Hippocrates, the ancient Greek physician who has been called "the father of medicine," described its symptoms for the first time. Ancient records from that period describe a flu-like epidemic that ravaged the Athenian army.

The origins of the term “influenza” can be traced back to Italy in the 15th century. Back then, they believed that the disease was merely a misfortune that was caused by the influence of the stars (“influenza” is Italian for “influence). In 1485, British medical records report of an illness whose symptoms are similar to the flu that takes the lives of thousands of people, including the mayor of Britain. At the time, doctors prescribe tobacco juice and lime juice to patients. They bled their patients to try to cure them.

In 1580, the first major flu pandemic in the history of the world is recorded. Estimates say it affected 90% of people worldwide and killed millions. The death toll was concentrated mainly in Asia, Africa, Europe and America. By the 18th century, people know the flu better and start calling it “influence of the cold,” which is translated as “influenza di freddo.” That’s how the flu was known to most of the world at the time.

The Spanish Flu struck hard in 1918 and would go on to become the worst pandemic in history. By most estimates, the Spanish flu claimed the lives of 50 million to 100 million people worldwide in 1918 and 1919. Contrary to popular belief, it was not called the Spanish flu because it originated in Spain or had an extraordinarily high influence in that country. It was the Allies of World War I who coined the phrase "Spanish Flu" simply because the disease received the most media attention in Spain than anywhere else in the world. The fact is that Spain was not involved in World War I and, as such, the country was spared from censorship. While the mortality rate for the Spanish Flu is not known, estimates indicate that it affected 20% of the world's population and killed 5% of all human beings. It was more lethal than AIDS. The Spanish flu claimed 25 million lives in 25 weeks while AIDS claimed the lives of 25 million in its first 25 years.

In 1957 and 1958, the Asian Flu broke out. It began in China but quickly spread around the world within a year and claimed between one million to four million people in two years.

Finally, in 1968, the Hong Kong Flu would strike mankind. It was the last flu pandemic to hit the world, killing up to 750,000 people in two years.

Although there were scares in New Jersey in 1976 (the Swine Flu), worldwide in 1977 (the Russian Flu), and in Hong Kong and other Asian countries in 1997 (Avian influenza), there have been no major pandemics after 1969, thanks to the development of effective flu vaccines that have limited the spread of the virus.

There is a common misconception that the common cold (stomach flu) and the flu are one and the same thing. They may be similar, especially with regard to the outward symptoms, but they are not the same. The biggest difference is that the flu can cause death. In addition, they are caused by different viruses. Gastroenteritis is called the "stomach flu" but that's a misnomer because it has nothing to do whatsoever with actual flu.

These days, influenza affects about 10%-20% of all U.S. residents each year, a number that will run in the millions. Most of these cases will recover in a week or two, but others will suffer life-threatening diseases like pneumonia.

However, the death toll is still frightening: 36,000 people per year die from influenza in the United States while between 250,000 and 500,000 people per year die from influenza worldwide.