You can read the whole letter here. An excerpt follows:
I have been long impressed with the same sentiments you so well express, of the growing felicity of mankind, from the improvements in philosophy, morals, politics, and even the conveniences of common living, and the invention and acquisition of new and useful utensils and instruments; so that I have sometimes almost wished it had been my destiny to be born two or three centuries hence. For invention and improvement are prolific, and beget more of their kind. The present progress is rapid. Many of great importance, now unthought of, will, before that period, be produced; and then I might not only enjoy their advantages, but have my curiosity gratified in knowing what they are to be. I see a little absurdity in what I have just written, but it is to a friend, who will wink and let it pass, while I mention one reason more for such a wish, which is, that, if the art of physic shall be improved in proportion to other arts, we may then be able to avoid diseases, and live as long as the patriarchs in Genesis; to which, I suppose, we should have little objection.
How apt Franklin's sentiments were regarding modern advances in medicine.