After the lethal second wave struck in the autumn of 1918, new cases dropped abruptly — almost to nothing after the peak in the second wave. In Philadelphia for example, 4,597 people died in the week ending October 16, but by November 11 influenza had almost disappeared from the city. One explanation for the rapid decline of the lethality of the disease is that doctors simply got better at preventing and treating the pneumonia which developed after the victims had contracted the virus, although John Barry states in his book that researchers have found no evidence to support this. Another theory holds that the 1918 virus mutated extremely rapidly to a less lethal strain. 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.