And, have you noticed any of the rescuers wearing protective gear? Any hazmat suits? I haven't. They just splash through the water (if they're in the flooded areas) and do their work.
They're not wearing protective gear, but I'm sure they have fresh water showers available if they're soak swim in that water. I have worked on both septic and storm sewers, and design and testing of dewatering processes. That water just doesn't approach septic water.
There's folks all over this country that have lived with overflowed septic systems and polluted waters. The waters were polluted, because the town put the untreated septic waters right into the Lake. It took a long time for folks to recognize how bad that was, because the sickness and death just didn't pop up in large numbers.
In this case, the folks wanting to stay in the interim expect to do so for a short time. This isn't a permanent settlement they're talking about. The scare mongers are also talking about tropical diseases that are already present and the extra water has no effect on their occurrence.