Influenza: Influenza: Introduction Influenza, commonly known as the flu, is a contagious respiratory illness caused by influenza viruses. It can cause mild to severe illness and, at times, can lead to death. The flu affects millions of people every year, leading to significant morbidity and mortality worldwide. History of Influenza Influenza has been known to humanity for centuries. Historical records suggest that flu-like illnesses have been around for at least several hundred years. The first well-documented pandemic was in 1580. Since then, the world has seen numerous flu pandemics, with the Spanish flu of 1918 being the most devastating, causing millions of deaths worldwide. Types of Influenza Viruses There are four main types of influenza viruses: A, B, C, and D. Influenza A and B are the most c...