Simply touching it is no big deal (in Argentina at least), but god forbid you stir the yerba, then you're in deep trouble.
Brazil's "chimarrão" (as they call the mate) is prepared pretty differently than we do in Argentina, and the customs are a bit different. They use way more yerba, a much bigger gourd and it's not as social (at least in Santa Catarina I very rarely saw people sharing the gourd.)
There's also "tereré", which is like "chimarrão", but it is infused and consumed with cold water instead of hot.
well that's because it makes all the powdery crap sink to the bottom instead of the big chunks acting like a filter
Paraguayans (at least the Mennonite colonists) don't have a problem with touching the bombillia either