EXOD574 [Regarding health and illness] A person should place his trust in the Creator, while seeking continued health through natural means. He should fight illness in the customary way—as the Creator, May He be exalted, has commanded: “And he shall cause him to be thoroughly healed” (Shemos 21:19)--yet have no faith in the power of the [natural] causes of health and illness to help or harm except with God's permission. When a person trusts in God, He will cure him of his illness by [natural] means or without such means, as it says: “He sends His word and heals them” (Tehillim 107:20). He may even heal him by means of something very harmful, as you know from the account of Elisha and the bad water, when he cured its unwholesomeness with salt Melachim II, 2:19-22). So it also says, “And God showed him a tree, and he threw it into the water [and the waters became sweet]” (Shemos 15:25); the Early Ones say that this was wood of the bitter oleander (Midrash Tanchuma, ad loc.) Similarly, it says, “Let them take a cake of figs and spread it on the boil, and he will recover” (Yeshayahu 38:21). You also know what happened to Assa when, upon becoming ill, he relied on the physicians instead of trusting in God-- how he was disciplined and reproved for this. Scripture also says, “He hurts, but He soothes; He wounds, but His hand also heals” (Iyov 5:18).