If you're on an Antarctic base where medical facilities are meager and the time and cost for evacuation is high, zero infections is likely the goal.
As an extreme example, the US grounded an astronaut for being exposed, not infected, by measles:
Medical evacuations are tricky, especially in the winter, and the logic is that the (single) doctor could remove someone else's appendix, but it would much harder for them to remove their own. Leonid Rogozov did remove his own in the 1960s, but I think most stations would prefer to avoid a repeat of that.
It did make for a fascinating BMJ Christmas Article though, written by his son: https://web.archive.org/web/20100925041337/http://www.bmj.co...
https://www.dailymail.co.uk/news/article-10349531/China-Covi...
I always thought the whole prevention thing is because we want to “flatten the curve”, remember? Not to prevent every single infection, which is impossible.