Is there a doctor onboard?

In the future, as told by Star Trek, medical science will have come a long way … but how far? And what will be the practical inplicatlons? First guess is that there won't be as many Doctors ... which I'm basing on the fact that the USS Enterprise seems to have only one doctor at a time - even though there are roughly 5,000 on board. (Hmmm ... I wonder what the doctor:patient ratio is in the general community in a modern country these days?)