As it is pointed out in the passage, water ----.

A group of biologists studying the habits of
chimpanzees around the Koba National Park, made
a surprising discovery. There was plenty of water
available for them in the pools left in river beds, but
these animals always liked to dig their own pools by
hand or with the help of sticks. As a result the water
they drank had been filtered through the sand and so
contained none of the disease-carrying substances
normally to be found in water that is not moving. In
fact, they were drinking clear water.