Influenza, commonly known as the "flu", is an acute viral infection of the respiratory tract caused by influenza viruses. Although it is often confused with other influenza-like illnesses, especially the common cold, influenza is a more severe disease caused by a different type of virus. A virus is a cluster of genes wrapped in a protein membrane, which is coated with a fatty substance that contains molecules called glycoproteins. Strains of the flu are identified according to the number of membranes and type of glycoproteins present. Although Influenza is not as severe as many viral infections it's almost the worst for viral infections of the respiratory tract. When you get the "flu " in the lungs, the lining of the respiratory tract is damaged by becoming swollen and inflamed. But the damage is not always permanent, and tissue heals within a couple of weeks. It is a respiratory disease, even though it infects the whole body. .
Every year, influenza strikes millions of people worldwide. Influenza epidemics are most serious when they involve a new strain, against which most people around the world are not immune. Such global epidemics (pandemics) can rapidly infect more than one fourth of the world's population. For example, the Spanish flu in 1918 and 1919 killed an estimated 20 million people in the U.S. and Europe and 17 million people in India. With modern society's dependence on air travel, an influenza pandemic could potentially inflict catastrophic damage on human lives, and disrupt the global economy.  Influenzas that occur every year are called "seasonal " flus.
Influenza is caused by any of several closely related viruses in the family Orthomyxoviridae (a group of RNA viruses). Influenza viruses are categorized as types A, B, and C. The three major types generally produce similar symptoms but are completely unrelated antigenically, so that infection with one type confers no immunity against the others.