Nobody directs the end of the world like George Romero did in the 70s. In a Romero apocalypse, everyone shouts at each other, no one knows what’s going on, blood sprays, heads explode, and life tips into outright surrealism. The world actually isn’t ending in this movie, but it may as well be for a small town just outside of Pittsburgh where a biological military weapon that makes people go insane was accidentally let loose in the water supply. Now the whole place is under quarantine and overrun with callous soldiers in gas masks who round up everyone by force. It’s a Vietnam-era/post-Kent State vision of the government and the military here. They’re schemers and bunglers and they’re happy to shoot us down when we get out of hand.
The film never quite sells us on the biological threat, but I don’t think Romero’s much interested in that. From the first scene, we’re lead to believe that it turns people into murderous mad dogs, but we later see some people just get a little loopy from it, like they just smoked some good pot, and it’s confusing (are they infected or just tired?). Romero puts most of his energy into scenes of infighting at the military nerve center and the efforts of a small group of people to avoid being captured or killed by trigger-happy soldiers. The bad guys here are the masked military men, not (for the most part) the infected people.
Romero made this entertainingly weird and caustic film in a period when his career was headed down the drain. He had two low-budget bombs before this, his production company was dying, and The Crazies ended up being his third financial failure. Over time, it found an audience among Romero cultists and got a big Hollywood remake in 2010.