I am very sad to hear this unexpected and unlooked-for news.  I first met Jan at the conference he ran in Groningen in 1983 on "priorities in the study of Indian medicine."  I was pretty wet behind the ears, and Jan's conference was a kind of intellectual baptism for me, into ayurveda studies.  Shortly afterwards, I had the privilege of collaborating with him in co-editing the proceedings of a follow-up meeting that took place at the Wellcome Institute in 1985 (Studies on Indian Medical History).  Both meetings were ground-breaking, and brought Jan's work to a wider circle as well as generating a new birth for Indian medical historical studies amongst European scholars.  

Jan's contribution to Indological study will continue to yield clarity and knowledge for decades if not centuries to come.  It may be less well known to colleagues in this forum that parallel with his professional life as a Sanskrit scholar and medical historian, Jan also conducted an active clinical practice as a psychiatrist, working for many years with some of the most extreme and disturbed patients in Holland. Further remarks on his multifaceted achievements will no doubt find place in formal obituraries.

I will miss him, both as a scholar and - as Ken Zysk has eloquently said - as a special person.

Dominik