Is common flu only a mild disease because past infections provide some protection?
Right. But there’s more to it than that. Some flu viruses are also inherently more deadly than others, for reasons we do not yet understand. Dangerous mutations can appear at any time, which is why some years the flu is much nastier than others. Luckily for us, viruses often evolve to become less dangerous as they circulate in one species. A virus that quickly kills every creature it infects tends to be less successful than one that leaves its host not only alive but well enough to walk or fly about and spread the virus to others. Descendants of the deadly 1918 flu, caused by an H1N1 virus from birds, circulated until 1957. But they became far less dangerous, partly because people developed immunity, but also because the viruses evolved into a milder form. In 1957 H1N1 was replaced by the H2N2 virus that caused the Asian pandemic that year. This also became milder and circulated until the H3N2 virus that caused the 1968 Hong Kong pandemic took over. The last time you got flu, a mild de
