Meningitus would be a major concern for me if sharing water bottles and it seems to have come to a head in recent times with the introduction of bottled water. Bottled water is unchlorinated so there is nothing there to disinfect bacteria and virus's from infected mouths, back wash etc.

There is vaccination available for meningitus but as usual the young population don't think about this and are the ones most likely to share drink bottles and other utensils when living in close quarters.
We have had a few recent cases again here of university students returning after the summer break and contracting meningitus with warnings coming from our Health Department to not share drinking bottles and get vaccinated. Meningitus can cause death within hours and treatment is intravenous high strength anti-biotics but even with this intervention death still often occurs. Very nasty virus to catch if away from good medical care.

A few drops of chlorinated laundry liquid or few grains of pool chlorine is perfectly safe to use in bush drinking water.