PhD to Medical Illustrator
This is what you need to do...
...get a 2 year masters degree
Advice from JI

PhD to Scientific Illustrator
No formal training yet, but a program is starting at Harvard
Guild of Natural Science Illustrators training path

Medical Illustrator to PhD
Expanding efforts and funding for hybrid technologies, bioinformatics, communication, education and outreach make this an increasingly viable option to initiate an academic career that expands the unique skill sets of medical illustrators.

Early career advice (high-school, college students)
Take general classes, explore all possibilities. If you're driven to both science and art, don't try to combine them yet, but explore both as deeply as possible. Graduate programs in medical illustration for example, are designed to train and fine tune talented artists who show talent for science- they prefer to fill and empty cup rather than un-teach what you may pick up in an undergraduate coarse in scientific illustration. There are, however, a few exceptions.  An excellent pre-medical illustration program at the University of Iowa, and an excellent undergraduate medical illustration program at RIT.

Useful Links