Question

Why might increased turbidity increase the chance of something harmful being in the water

Answer 1

In drinking water, the more turbid it appears, the higher the chance that  those using it could end up with  gastrointestinal diseases.  The water may be having contaminants such as viruses and pathogenic bacteria which are found attached to suspended solids. These solids may interfere with disinfection of the water.

Erosion and effluent from highly urbanized  areas contribute to the turbidity of water in those areas.  In open bodies of water, organisms such as phytoplankton can contribute to turbidity in the water.