Malaria has infected humans for over 50,000 years, and Plasmodium may have been a human pathogen for the entire history of the species.  References to the unique periodic fevers of malaria are found throughout recorded history, beginning in 2700 BC in China. Malaria may have contributed to the decline of the Roman Empire and was so pervasive in Rome that it was known as  “Roman fever". The term malaria originates from Medieval Italian: mala aria — “bad air”; the disease was formerly called ague or marsh fever due to its association with swamps and marshland.  Malaria was once common in most of Europe and North America, where it is no longer endemic, though imported cases do occur. Malaria was the most important health hazard encountered by U.S. troops in the South Pacific during World War II, where about 500,000 men were infected. Sixty thousand American soldiers died of malaria during the North African and South Pacific campaigns.