I can't find anything on the Internet to support the theory that greeting by way of Namaste was practiced for hygiene reasons. Do you have any citations for that?
I think galtwho is saying that the people who said Namaste instead of shaking hands were more likely to pass on that greeting as they were less likely to die of a handshake illness. I.e. it came about through natural selection, rather than because someone had decided it was healthier.
On reading galtwho's comment again, I feel you are right. I thought he/she was repeating the post-hoc reasoning that is common in India, where people would take a ritual and assert that ancient Indians knew about scientific ideas that are common knowledge today and based the ritual on those ideas.
