It wasn t until the 15th and 16th centuries, however, that there was clear evidence of its spread among humans. The origins of influenza are unknown, but the malady is never absent for more than a few decades. The worst pandemic occurred in the late 19th century, when more than 20 million people were killed. Several possible identifications have been suggested, including influenza, whooping cough and diphtheria. The historian Livy describes an infectious disease, perhaps influenza, which strikes the Roman army.
Influenza also spreads widely in North America, the West Indies and Spanish America.
Most deaths appear to come from secondary pneumonia. From there it spreads to the Philippines in September 1830, to Indonesia in January 1831, through the Malay peninsula and into Asia in 1832. By November it reaches the United States, and continues to spread in 1832. Named the Russian flu, this worldwide influenza epidemic, the most devastating to that time, begins in Central Asia in the summer of 1889, spreads north into Russia, east to China and west to Europe. It eventually strikes North America, parts of Africa and major Pacific Rim countries.