The Cyclothymiac
An infected with a dual moveset.
Manic
While in manic, it struts towards survivors flailing its arms, and laughing a cracked crackle, interspersed with gargling coughing. Its attacks consist of a lunging grab, constricting the survivor. Due to high HP, it is difficult to remove, so the best option is to avoid it until depressive. While neutral, it will often stumble down streets wringing its hands and twitching.
Depressive
The infected starts in Depressive. While in this form it will have incredibly low endurance, meaning the high HP is not as useful, as it decreases quickly. While depressive, it will not make any noise whatsoever, and will function like a Witch without the warning. While like this, it is often prone, perhaps lying motionless on the ground. The only indication of it spotting a survivor will be a slow intake of breath.
Appearance
It should somehow reflect its dual personality. Perhaps one side of its face could be twisted in a perpetual grin, and the other constantly gloomy. Perhaps the hair? I don't know, I'm bored and probably have something else to do. Someone else do it.