Ditto yaah.
Might want to ask in one of the other forums for more detailed information about the life of an infectious diseases physician. As for medical microbiology (using yaah's basic description of primarily a laboratory specialist), it's a relatively small niche. The MD's coming at it from the pathology route (rather than a non-MD PhD-only, who I get the impression are competing well for those jobs) are fairly few and far between; in 2009 11 people took the subspecialty exam and 9 passed, and it's only offered every 2 years. Maybe one of them is lurking around and can comment.
My impression, though, is that unless you're a gun &/or willing to find a large &/or academic lab, it's not easy to find a job doing nothing but medical micro while getting paid reasonably and actually maximizing the use of fellowship training. Mix it with other things, however, and that changes (also cover clinical chemistry, the rest of the lab, or some AP, as examples). Kinda the classic issue of whether to pay a PhD less to do one job, vs an MD more to hopefully do more than one job.