After yet another apparently spontaneous sex scene, I wondered "Geez, don't these people do anything about birth control or STI's?"
Then I realized the answer.
"There's probably a mod for that."
There are probably injections for long-term contraception, much as Sisko and Kasidy alluded to late in DS9. And there are most likely immunizations or easy treatments for STDs (or whatever they're calling them now). But yes, there are mods for enhanced immune systems, biological or nanotech (or a mix of both).