The start of the movie has small town veterinarian investigating sick animals (horses?). Then humans become sick. It turns out the water supply is toxic. The vet survives because she is a bottled water fanatic and doesn't drink tap water.
Authorities quarantine the town. The vet confronts their leader who gives her a pitcher of water to drink ... and of course it's the contaminated water.
She escapes; the film ends with her dying of the plague near a road that leads to a big city and it's implied that a family in a car are infected and en route to this city.
I think it's a post 2000 movie but I'm not sure.
It's not The Crazies.