This is a movie from around 15 years ago. There's a cabin in the forest. A stranger asks for help and uses bathroom. He emits a horrible smell and dies after ejecting a worm-like alien. Aliens attempt to take over earth through water system.
In the end, an alien is headed for an opening in the floor of the water treatment facility in, Boston(?). Just as it approaches the edge, one of the two good guys from the cabin squashes it with his boot.
This is Dreamcatcher (2003).
One night, Jonesy sees Duddits beckoning him to cross the street, but as he does so, Jonesy is hit by a car. His injuries heal with mysterious speed, and six months later, he is able to make it for the group's annual trip. Jonesy rescues a man lost in the forest named Rick McCarthy. He is very ill, so Jonesy and Beaver let him rest and recover inside their cabin. Suddenly, all the forest animals—predator and prey both—run past their cabin in the same direction, followed by two military helicopters that announce the area is now quarantined. Jonesy and Beaver return to the cabin to find a trail of blood from the bedroom to the bathroom, where Rick sits dead, covered in blood. A three-foot long worm-like creature writhes and screams in the toilet. Beaver sits atop the toilet lid to trap the creature, but the creature breaks out and kills him. Jonesy tries to escape, but is confronted by a large alien called Mr. Gray, who possesses Jonesy's body.
... [At the end]
Jonesy, now himself again, steps on the final alien larva before it can contaminate the reservoir.
