Turning into a 'stealer? Genestealer infection has no external signs of symptons, at least until the first mutant child is born...
The 'how' is a question left open, though the term 'implanted' has been used. My theory is that some weird 'nid organ literally is implanted, and whether it remains within the body forever, releasing hormones to mess with the DNA of zygotes, or dissolves away after changing the genome of some cells is up for debate.
I don't know if 'stealers bother with infection during invasions (I may be wrong), as there seems little point infecting someone you're just going to kill and absorb the biomass of at a later date. During an invasion, they're just a particularly nasty member of the great swarms and only as advance scouts do they revert to their better documented Xenomorphic behaviours. I may be wrong, though, as I've not seen the latest fluff and know very little about Broodlords.
Story needs to sort out that issue, Swarbie, but it should be interesting. Go for it. They're interviewing survivors for the Strategic Collective - they can grant a few days stay of execution for that.