THRESHOLD Episode 11 "Vigilante"

Ramsey is enjoying a tryst with a stripper when he is rudely interrupted by an alien, but one that kills the stripper and leaves. It turns out that the stripper was an infectee and the man who killed her is a doctor who is only partially mutated and so is able to detect the aliens and kill them, but is not able to distinguish between full infectees and those that have had low levels of exposure such as Molly, Cavvanaugh and Lucas. Information also comes in about the location of the infectees' leader and it appears that he is now after the water.