Cujo is a St Bernard dog owned by a mechanic, Joe Camber (Ed Lauter), who lives out of town in the countryside, which explains how the hound has the opportunity to chase rabbits as he is allowed to wander the fields as he wishes. But he makes a mistake following this particular animal, which races across the river and into a cave; Cujo pushes his nose through the small hole to find not the rabbit but a colony of bats, which panic when he barks and bite him on the nose. And bats very often carry rabies...
Stephen King's novel Cujo was written at the height of his alcoholism, and he said he could not honestly recall writing it such was his battle with the bottle at the time, but for a man typing away while drunk or otherwise inebriated it certainly didn't read like an author out of control through his addiction, if anything it was as keenly plotted as any of his novels, more than some of them in fact. Not that this is an endorsement of heavy drinking to get those creative juices flowing, but it might explain why King was so keen to make a movie of the book, so he could change one aspect he considered a mistake.
I can't reveal what that was without spoiling both book or film, but that ending was not really an error in either, as they both operated for the good of the material in each medium. The plots were basically the same, and the screen version was a model of economical adaptation from page to movie, keeping just enough of the source to sustain the tension and essential concerns. The reason why the book was so unsettling was not because it was the typical King supernatural threat he used so often, but because it illustrated how a collection of events, some big and some small, can conspire through coincidence or ill fortune to create tragedy.
Or horror. The potential victims are Donna Trenton (Dee Wallace) and her young son Tad (Danny Pintauro), and they happen to end up at the Camber farm when she wishes to get her car fixed, but even before we got to that stage, we had to learn some background and understand what made these people tick. The first attack by Cujo, now rabid thanks to that bite, actually occurs at the halfway point of the story, meaning that many audiences were restless with regular cutaways to the dog whining, whimpering and growling as he grew progressively dirtier thanks to the disease raging through his system.
Especially when the details of Donna's dilemma seemed the stuff of drama rather than chills: she is having an affair with Steve Kemp (Christopher Stone, Wallace's husband at the time), who is the tennis partner of her husband Vic (Daniel Hugh Kelly). To make matters worse at home, just as Vic's advertising account backfires when the product causes temporary sickness, he finds out about the affair and doesn't much care that his wife insists she still loves him and wants to save the marriage, so off he goes to rescue his account in New York for a few days cooling off. Thus when Donna and Tad go to get their faulty car fixed, no one knows where they've gone, not good when the vehicle breaks down, Cujo kills the only two people about, and lays siege to them both in their increasingly stifling car. Efficient was the word for the film, and director Lewis Teague worked up some truly intense dog attacks to ramp up the tension, nothing spectacular, but a decent adaptation that did no disservice to the novel. Music by Charles Bernstein.