Can you help me find a certain horror movie?

I’ve been looking for this one horror movie forever, but the problem is, I obviously don’t know the title, and I only remember one specific scene. I saw this when I was little, but I was watching it on and off. Some scenes stayed in my head though, but I’ve never been able to find it again.
I don’t have much of a lead. I’m pretty sure the movie comes from the eighties.
The premise of the movie is a virus, or bacteria or something in drinking water. Those who drink the water turn into grotesque mutant versions of themselves who run around killing people. It occurs in a small community, I think our main characters live on a farm.
Two scenes I remember; one is an older brother who turned into a psycho, and he’s holding his little sister over his head. He’s about to throw her, but something happens which prevents this. I don’t remember what. I think he gets shot by some dude with a rifle. He’s also puking all this orange stuff.

The other scene is a man and his son arguing about whether or not to stay on the farm. One wants to leave, but the other wants to stay. The one who wants to take off has tin cans of water with him.

That’s all I got. Does it ring a bell to anyone at all? Thanks.