This video from the National Institute of Allergy and Infectious Diseases discusses the causes of influenza pandemics. When 2 different influenza A viruses infect a single animal, the narrator explains, they can combine to form a new virus to which people have limited immunity. There were 3 influenza pandemics during the 20th century, in 1918, 1957, and 1968. The first pandemic of the 21st century, the H1N1 pandemic of 2009, is thought to have occurred due to 2 viruses circulating in pigs that created a new virus capable of infecting people.
The current measles outbreak across 14 US states indicates that too few individuals are being vaccinated to prevent infections, according to Jason Gallagher, a clinical pharmacy specialist in infectious diseases at Temple University.