George Romero's The Crazies involves a biochemical warfare virus code-named Trixie that gets into the water supply of Evans City, Pennsylvania. It has two equally unpleasant effects, either killing its victims outright or driving them hopelessly insane.
The military descends on the town like a plague of locusts, quarantining the area and dragging the frightened citizens from their homes to be corralled at the local high school while the powers that be figure out what to do.
Human interest revolves around firefighting Nam vet David and his pregnant wife Judy, who try to escape the quarantine, the virus, and the militant redneck locals whom Romero portrays as even more fearsome than the soldiers.
There's also an infected father and daughter, played by Richard Liberty (Day of the Dead) and pretty Lynn Lowry (Shivers), who gives the film's best performance as an innocent waif who mourns the passing of her own sanity.