How? It’s a vacuum! That said it’s thought there are several micro-organisms tough enough to survive dormant in space for ages. Fred Hoyle was an astronomer who developed this into a theory of the origins of life, well worth a read.
It’s hard to say. Nothing that we know causes disease (viruses, parasites or bacteria) has been found in space yet and there’s no planet that we know of where these could live in the forms they take on Earth. However, there could be other things which would make us ill which haven’t been found yet!
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