The problem here is that Haiti is very poor, and think that having a drinking water facility without the proper filtration should be an excuse to blame someone else for bringing in germs, viruses or bacteria. The thing to remember is if your infrastructure is badly built from the beginning without having any contingency plans setup, you will get burned, and it will only be your own fault.
If i go on a trip and have only enough fuel to make it to a specific place, and then stop to pick up a hitchhiker, adding more weight to my car, leaving me to run out of fuel before the next gas station, should i blame the hitchhiker? No one forced you to pick him up. If you were smart enough to have a gerry can in the back with just a bit more fuel to get you to that gas station, which most people do before long treks, you avoid this situation.
Having one source of drinking water with no special reserves setup and no proper filtration in place to catch all the contaminants, leads you to have this situation.
Dont blame someone for helping when you should have helped yourself before they got there. Just refuse the help next time if you are concerned they will bring disease with them. No one forced you to take their help.
As for the UN, there should be more testing in place for who they send in, this could easily have been avoided as well from the other side of the coin. We had firefighters and cops from here go and help over there, but i can guarantee you no one tested them properly for any sort of communicable diseases.