Introduction

Influenza - commonly called the "flu" - is one of the oldest and most common diseases known. It is an acute, contagious respiratory tract infection, which is caused by one of the influenza viruses. Flu causes illness worldwide, and sometimes with devastating effect when it occurs as an epidemic. It can create potentially deadly complications such as pneumonia. Influenza was first described by the Greek physician Hippocrates in 412 BC and the first well-described pandemic of influenza-like disease occurred in 1580. Since that time, 31 such possible influenza pandemics (worldwide outbreaks) have been documented, with three occurring in this century: in 1918, 1957 and 1968. The disease today still affects large sections of the population each year. In modern times, outbreaks of influenza happen nearly every winter with varying severity. Most outbreaks occur between October and May, with peak intensity from late December through early March.

Its ability to kill stems from the fact that the virus can mutate quickly, often producing new strains against which human beings have no immunity. When this occurs, mortality from influenza can be staggering such as, for example, during the "Spanish Flu" pandemic of 1918-1920, when at least 20 million people died from influenza.