Why don't Starfleet crew hardly ever succumb to local illnesses when they make planetfall? Why don't they get sick from the food or water? Why don't they ever kill off local populations like the colonists unwittingly killing off the Indians? Are various species magically immune to each other's illnesses, viruses, bacteria, bugs? Is this addressed?
You know what's a surprisingly fascinating virus? It's the virus that causes warts. Well, I say 'the', but that's a misleading article. There are many viruses which cause warts, and here's an astounding thing about them: a virus that gives you warts on your face will do very nicely there, but struggle to take hold on, say, your fingernails, or your feet.
But then the species of virus that will populate your feet with warts, if transplanted to your face, will struggle mightily. There's kinds that will grow warts around your finger- and toe-nails, but not really do much anywhere else. They're all wart viruses, all recognizable as such, and yet, they're so well-evolved and so well-adapted that not only are they optimized to a single species but they're optimized to different parts of the skin of that species.
As we lack experience with truly alien biochemistries, we don't really have the evidence to say whether diseases which have evolved to target the life in one biosphere will do well, or do nothing, or do something strange, on life from another biosphere.
I admit I would find it plausible for, say, Vulcanian influenza to do poorly attacking humans; it can be hard enough for one bug to attack a new species here on Earth. But I wouldn't rule it out, either on scientific or on dramatic grounds.
