That's a much rarer course and seems to be related to the conditions of WW1. Under normal circumstances, people with mild illnesses are out and about and able

That's a much rarer course and seems to be related to the conditions of WW1. Under normal circumstances, people with mild illnesses are out and about and able to spread the virus, while people with severe illnesses are confined to a bed or hospital and tend to result in an evolutionary dead end. As a result, the descendants of mild cases outcompete the descendants of severe cases, and eventually natural selection results in the dominant strains of the virus all being mild. With the 1918 flu pandemic, mild cases of flu remained with their unit, while extremely sick cases were transported home via train for improved medical care. As a result, it was the severe cases that ended up sparking new infections, while the mild cases died out after generating herd immunity within an army company. Most respiratory viruses follow the first scenario, although there is a caution there for people who suggest eg. transporting sick people by airliner to regions with fewer cases to make better use of available hospital capacity. Ironically asymptomatic transmission is your best friend here: if all transmission is asymptomatic, the strains that get transmitted will be the ones more likely to not cause symptoms.