An influenza pandemic occurs when a new influenza strain to which humans have no immunity emerges, mutates and spreads globally. Three influenza pandemics have occurred during the 20th Century. The deadliest was in 1918 and resulted in as many as 50 million deaths worldwide. The most recent was in 1968 and caused about 700,000 deaths around the world. During the last four centuries, an influenza pandemic has emerged about once every 30-40 years.