Problem with that is, EVERY planet would present itself with a biological contamination problem because every planet would have bacteria, viruses and other infections on it humans are not immune to due to lack of exposure. So we assume that there's ways around that, either through advanced medical techniques and the biofilter on the transporter. I think the series would get pretty boring if every other week we had characters near death from an alien cold they're body doesn't know how to fight.
Lol, true. You wouldn't want to see that every week. But it would still be nice to bring in at least a LITTLE more reality to the exploration we see in Trek, I think. Instead of having every planet be clean and antiseptic, and comfortable enough where they don't even have to wear a freakin jacket.