influenza viruses

The 1918 flu pandemic (the "Spanish flu") was an influenza pandemic, and the first of the two pandemics involving H1N1 influenza virus (the second was the 2009 flu pandemic, an outbreak of swine flu). It was an unusually severe and deadly pandemic that spread across the world. 
The pandemic lasted from June 1918 to December 1919,^[3] spreading even to the Arctic and remote Pacific islands. Between 50 and 100 million died, making it one of the deadliest natural disasters in human history.




This is a common occurrence with influenza viruses: there is a tendency for pathogenic viruses to become less lethal with time, providing more living hosts.^[10]