Twilight

A major point of this episode is that when the parasites are killed, they vanish from history and so the past is altered - things like medical scans appear as if the parasites had never existed. But that would mean that the memories people had of those medical scans would also be re-written, so T'Pol and Phlox would never know anything had happened. In fact, Phlox could never even know his treatment had worked at all!

In fact an interesting thing should have happened here. Phlox picked a parasite to test his treatment on. When it worked and the parasite was erased from history, in the new reality Phlox would have picked another parasite instead. So that one would then vanish from history and make another new timeline. In this one Phlox would pick yet another parasite... and so on. The upshot is, as soon as Phlox did his first treatment ALL the parasites should have vanished and the episode would be over! But if the parasites had never been there, why would Phlox invent the treatment in the first place... time travel makes no sense at all, does it?

The whole planet Earth and most of the Human race! We also see several ships destroyed, including Enterprise.