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Not all water is safe ...
Bilharzia or Schistosomiasis in humans is a chronic disorder caused by small parasitic flatworms of the family Schistosomatidae.  They live in the blood vessels of man and other mammals, producing eggs that cause tissue damage.  Tow species of bilharzias inducing flatworms are found in Africa. Schistosoma haematobiuma nd S. mansoni.
The female worm, when living in the blood vessels, lays between 300 to 3500 eggs daily into the blood. These eggs find their way into the intestine or bladder and eventually into fresh water sources.
On contact with fresh water, the eggs hatch, releasing ciliated larvae that swim about until they find an appropriate snail host in which to develop further.  After completing another stage of their life cycle, fork-tailed larvae called cercariae, subsequently emerge from the snail into the water.  Upon contact with the skin of a mammal, they penetrate the tissues, and so another cycle begins.
The clinical symptoms of the disease begin with allergic reactions to the parasites, and gradually the flukes and the eggs build up in the body, leading to serious lever damage, kidney stones and lesions in the brain and lungs.  During autopsy, fluke eggs have been detected in almost all body structures.
Unless exposure is overwhelming, early diagnosis and persistent treatment usually ensures recovery. 
Schistosomiasis is contracted by bathing or drinking water populated by the snails that carry the worms.  It is more prevalent in rural communities where the standards of hygiene are low. 
It has been found in the streams, dams and valleys on either side of the Magaliesberg.  The only really safe water is fast flowing water or water that flows on the top of the mountain range.