He went to Benares and made his tests.

He got water at the mouths of the sewers where they empty into the river at the bathing ghats; a cubic centimetre of it contained millions of germs; at the end of six hours they were all dead.

He caught a floating corpse, towed it to the shore, and from beside it he dipped up water that was swarming with cholera germs; at the end of six hours they were all dead.

He added swarm after swarm of cholera germs to this water; within the six hours they always died, to the last sample.
Repeatedly, he took pure well water which was bare of animal life, and put into it a few cholera germs; they always began to propagate at once, and always within six hours they swarmed--and were numberable by millions upon millions.
For ages and ages the Hindoos have had absolute faith that the water of the Ganges was absolutely pure, could not be defiled by any contact whatsoever, and infallibly made pure and clean whatsoever thing touched it.

They still believe it, and that is why they bathe in it and drink it, caring nothing for its seeming filthiness and the floating corpses.
The Hindoos have been laughed at, these many generations, but the laughter will need to modify itself a little from now on.