10-15 years ago (I was pretty young, maybe 13-14) I got my hands on a book about this space expedition on some celestial bodies that were actually alive and dormant. The book was in English, it might be 30+ years old. I remember it was pretty short, 100-150 pages and (I might be mistaken) but it was part of a collection of space-travel short books by the same author.
I don't remember much, as I didn't get to finish it, but I remember the story was told from the perspective of these spacemen that land on this (these) creature(s) that float through space, not knowing they are alive. The interesting part is that they get contaminated through the ground(skin) of the creatures and their DNA is mutated according to the area they landed on. I might be wrong on some of the details, it was a long time ago and my memory is terrible.
Also, about the story, I remember there was a pretty weird part involving growing genitalia because of the influence of the space-creature-thing. I remember because I was pretty embarrassed reading that part.
EDIT: I found it! I tracked down the friend who I got it from and she found the guy she borrowed it from and in the end we found the book: it's Serge Brussolo - Territoire de fievre. Apparently it was french so I threw you off quite a lot. Thanks for all your efforts!