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It's also very normal for viruses to become more mild over time. We strongly react to strong threats. It's in the viruses interest to become a weaker threat li

It's also very normal for viruses to become more mild over time. We strongly react to strong threats. It's in the viruses interest to become a weaker threat like the common cold so we tolerate it instead of actively fighting it.

jeremyjh 7y ago
They can also become much more deadly. The first pass of the 1918 pandemic was not nearly so bad as the second, which slaughtered millions of people in their 20s and 30s.
- AnimalMuppet 7y ago
  True. But I have heard that that was because, among all the soldiers in the trenches, it could be virulent and still be able to spread effectively, so it had no selection pressure to not be virulent. Covid is a bit different, because it has a longer incubation time, but that wasn't the issue with the 1918 influenza outbreak. (Note well: I am not an epidemiologist.)
- nostrademons 7y ago
  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.

jeremyjh 7y ago

They can also become much more deadly. The first pass of the 1918 pandemic was not nearly so bad as the second, which slaughtered millions of people in their 20s and 30s.

AnimalMuppet 7y ago
True. But I have heard that that was because, among all the soldiers in the trenches, it could be virulent and still be able to spread effectively, so it had no selection pressure to not be virulent. Covid is a bit different, because it has a longer incubation time, but that wasn't the issue with the 1918 influenza outbreak. (Note well: I am not an epidemiologist.)
nostrademons 7y ago
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.