The possibility of non-carbon based life is so remote as to be irrelevant. We'd have to go through many star birth and supernovae cycles to get to the point where silicon is even anywhere near as abundant as carbon is now.
As for microbes from other worlds, who knows. The properties of water are pretty much required for life (as far as we know) and so they'd have 0 trouble interfacing with most organisms. Combine that with the fact that the basic amino acids can form in a wide range of conditions and you've got the recipe for interference. Also, infections happen when organisms invade and multiple, some times the damage is a secondary side effect (i.e. waste or toxic byproducts)
As for microbes from other worlds, who knows. The properties of water are pretty much required for life (as far as we know) and so they'd have 0 trouble interfacing with most organisms. Combine that with the fact that the basic amino acids can form in a wide range of conditions and you've got the recipe for interference. Also, infections happen when organisms invade and multiple, some times the damage is a secondary side effect (i.e. waste or toxic byproducts)