The Mishna teaches that on Rosh Hashana, the entire world comes in judgment before Hashem. The decrees that result from uncorrected sins are liable to manifest themselves in sickness. But there's hope...
The Gemara teaches (tractate Avoda Zara 55a), that "immediately before disease and suffering are cast upon a person, they are sworn to leave on a certain day, at a certain hour, by a certain person and by a certain medicinal agent." Rebbe Nachman of Breslev teaches (Likutei Moharan II:3) that all the above conditions must be met for a sick person to be cured. If so, how can a physician cure anyone? A doctor cannot possibly cure a person unless he or she is the designated Heavenly messenger with the proper cure at the proper time. Even more so, how does a sick person seek a doctor’s help if he or she doesn’t know if that particular doctor happens to be the designated messenger? As such, going to doctors is like betting on horses – maybe you’ll win and maybe you won’t.