Could also be Cronenberg's "Rabid" (from 77' I think).

There's a film from 2010 that's really really similar, called The Crazies.

http://blankingdelicious.tumblr.com/
Replace "blank" to get the real URL.

I believe the movie Warning Sign from 1985 is another one similar to The Crazies.

I've been dying to see Impulse again.

"Keep the Ancient Flame still Burning"

There's another movie, with a small midwestern town exposed to some sort of toxin that reduces their inhabitions in a very dangerous way( imagine actually running over that little old lady that takes an hour to cross the street, instead of just thinking about it): I think it was called Impulse?
"Yes, it's the Apocalypse alright. I always thought I'd have a hand in it."

or it could be "Nightmare City", fits the description nicely

The film you're talking about is "Rage" from 1972. It starred George C. Scott. I loved the movie too and I still watch it every few years. I have a great copy of it on VHS and I really enjoy it. "Rage" also has anti-military themes similar to "The Crazies." I loved both of these films.

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