I got a theory but I might be totally wrong, or people have already talked about it

I noticed today that a British officer called runners the "fresh ones."

I think this might suggest that a viral runner, or a runner, is in the earliest stages of infection, one whose muscle function has not yet deteriorated and can run very fast. This explains why players turn into a viral zapper, curassier, or a viral runner instead of a shambler. Shamblers are those who have been infected for a while, whose muscle tissues began to decay after about a week, so they couldn't run.

I thought that, in the beginning, the infected were mostly runners. Perhaps this is the reason why so many places fall: they are highly mobile and can quickly infect many people.