> I've read some where the common cold was probably a pandemic virus originally, and it mutated to a common cold we have today, and we mutually evolved to deal

> I've read some where the common cold was probably a pandemic virus originally, and it mutated to a common cold we have today, and we mutually evolved to deal with the common cold. Something like that... A factor that comes into play is the following. Diseases tend to be less dangerous to children. When a new disease emerges, no-one has immunity to it, so there's plenty of adults who may get it more severely. But, after a certain point, the people being born are exposed to that disease when they are young, where it won't affect them as badly. This can look like the disease getting milder, but it's just that after a while most people will experience it when young when they will get milder effects from it. And after that point they'll have some immunity for it.