Almost universally, yes. And with most, it's generally chemistry or biology, although a few biophysics and bioethics programs are out there. If you want to do social science work, then you get a Masters of Public Health.
none is correct for the most part. There are a couple of exceptions, most notably the MD/PhD program at UIUC (Univ. of Illinois at Urbana-Champaign). They will let you get a PhD in ANY field you want to, whether its literature or immunology. However, there is a downside to UIUC since its a non MSTP program (no NIH funding). So they will only give you financial support at the half-time (graduate) and quarter-time (medical) levels. I think there might be a couple of other schools that have non MSTP programs like UIUC who let you choose your PhD from any field. But they are few and far between.
