Other mistake: After they kill the alien/teacher and leave the school they pass one girl in a blue/green tee-shirt with a waterbottle. Just one scene later you see this girl again under a tree, suddenly in front of them.
Other mistake: When Zeke is examining the alien, he puts his glasses on to look down the microscope, but wears them at no other point in the film. Since microscopes can be focused to the eyes of the user, most scientists take their glasses off to use a microscope.
Other mistake: While Ms. Burke appears fine at the end despite losing her head, Jon Stewart still has the eyepatch from earlier when he should be healed too.
Answer: The aliens needed fresh water. All the infected were drinking bottled water. When the gang were examining the slug in the garage, it dried up and died from salt, like a snail.