I won't bother trying to convince people to follow my beliefs (or lack thereof), since you can't convince anyone but the most gullible people on earth with one sentence. So I would just try to dispense advice.
My sentence would be:
"Please, please, please, for the love of whatever deity you worship, if you have the water to spare, wash your hands before you eat!"
From a public health standpoint, it's still invaluable advice.
I suppose another sentence could be:
"If you're overweight and diabetic and your toes are constantly numb or tingling, go see a doctor right away". It's really important to know that, and so many people have diabetes these days.