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Civilization has a tendency to flock to natural water sources. People are going to go to water, not to food. If you can go to food, and draw your own water from the earth, you're ahead of the game.
Also, if you intend to "develop a vaccine" to put in a water supply in bulk, you'll observe how quickly it becomes ineffective. There are probably hundreds of true vaccines. All of them are derived from nature. As a result of that, every vaccine that exists, has a complementary resistance that has been evolved as well.
Why? Because we aren't at the point in molecular biology where we can create truly effective vaccines yet. We're just borrowing from the arms race between fungi and bacteria.