How come the water is not infected?

[nikvoodoo]

I would guess that whatever the infectious agent is, it does not survive outside of the host organism very long but is passed only by direct contact. The survivors have been in close contact on so many occasions but only infected by direct injury.

There is no evidence (yet) that says how long the pathogen (to allow for more possibilities than the word virus grants us). Tommy got the Jumpers guts sprayed all over him and about five minutes later it gets into his wounds and he turns. For all we know, the pathogen can live outside a host body for years until evidence is produced to the contrary.