Somehow they figured it out without ever hearing about germ theory

In Nepal I was told that people liked to use copper and brass utensils for their healthful qualities.  What these exact qualities consisted of was not made explicit but faith in the concept was strong.  People believed that food stored in brass was rendered more potent somehow.  Well, it turns out that faith and folklore were solidly grounded in empirical evidence.

With the increase in resistant bacteria and the incidence of infection in hospitals there have been a number of recent studies in Japan, the United States and Europe of the bacteriological performance of different surfaces.  It turns out that both plastic and seemingly sterile stainless steel are surfaces on which bacteria, including MRSA and the deadly E-coli 157 can live for long periods of time.  Surprisingly, the best performing surface was copper (well nobody included arsenic surfaces which would probably really do the job).  One study showed E-coli living on stainless steel for 34 days while surviving only 4 hours on copper.  Brass, which is mostly copper with a bit of zinc also performed admirably.  You may well begin seeing brass table tops, food preparation tables and unvarnished brass doorknobs popping up as functional features in institutional settings.  The VA is currently doing a field test to the idea in one of its hospitals.

In Tibet Jambati bowls holding water over time would have the ability to vastly reduce the amount of pathogens in the water while Manipuri bowls in the kitchen would help with food safety in that setting.  The idea that copper and brass were especially healthy is also a part of European folklore.  So the existence of millions of brass bowls in the Himalayas is really not that hard to understand.  How some of these bowls come to sing so beautifully, well, that remains a mystery.