Three influenza pandemics struck the world in the 20th century, including the Spanish flu of 1918 that claimed anywhere from 50 million to 100 million lives. (There were no effective flu vaccines available at the time.) When a flu that contagious spreads across the world, how does it ever die out?
It runs out of victims. Infectious diseases like the Spanish flu spread exponentially as more and more people are infected and become contagious. As people develop immunities, receive vaccines, or otherwise shield themselves from infection, the pool of possible victims dwindles until the virus can no longer sustain itself.
Epidemiologists often describe the rate of infection in terms of a reproduction number, the average number of new people whom each sick person will infect. If this number is higher than one, even by a small amount, the disease is still spreading. (One study estimates that the reproduction number of the Spanish flu was 1.49 when the disease first hit Geneva and a whopping 3.75 in the second wave, which came shortly thereafter.) If the number is less than one, the disease is on the decline.
When a significant portion of the population has become immune to the strain of the flu currently going around (through either exposure or vaccination), it's much harder for a virus to maintain a high reproduction number. During the Spanish flu pandemic, as much as one-third of the world's population had symptoms of the disease, and many more were probably exposed. Since only about 2 percent of those who got the flu died, a large proportion of people became more or less immune. (Flu viruses are constantly evolving, but chances are you won't catch a slightly mutated version of the same virus you recently had.) So while the Spanish flu was, in theory, limited only by the number of people on the planet, it eventually ran out of nonimmune, nondead people to infect.
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Explainer thanks Lauren Ancel Meyers of the University of Texas-Austin, David M. Morens of the National Institutes of Health, Trish Perl of the Johns Hopkins University School of Medicine, and Gerald F. Pyle of the University of North Carolina-Charlotte.