Pool of Fresh Water not what it seems

Polluted water is more like sewage, fresh water means water that is relatively clear and does not have salt in it but it does not mean there are no germs. Thats how the term fresh water is used in real life.

polluted water is more like non-living contamination like toxins and chemicals. Germs can still inhabit "clean" water.

If anything,  this might be a map generation issue though. Normally slime lung only seems to shows up in slime biomes.

If the water is touching slime it can be infected.