In colonial America, your minister often provided succor to those afflicted in faith and fever. Ministers were usually the most educated people in a community so it made sense that they would be entrusted with medical care as well. Not to mention that so little was known about disease and its causes that fusing religion and medicine made as much sense as any other theory (and probably more so to the deeply religious American colonists). As historian Richard D. Brown put it, "even the most rational and learned individual..saw no clear boundary between physical and spiritual phenomena." The joining of these two roles, however, was not without its complications.
Cotton Mather, one of the most prominent Puritan ministers in New England, introduced smallpox inoculation in 1721, a move that ignited a fierce debate in the Puritan community. Puritans believed that every affliction was proof of God's special interest in their affairs. Some suggested that smallpox (a serious and deadly threat in colonial America) was perhaps God's punishment for sin and that to interfere with that through inoculation or anything else would only anger God and provoke more punishment. Mather did not waver, though. He wrote that "whether a Christian may not employ this Medicine (let the matter of it be what it will) and humbly give Thanks to God’s good Providence in discovering of it to a miserable World; and humbly look up to His Good Providence (as we do in the use of any other Medicine) It may seem strange, that any wise Christian cannot answer it. And how strangely do Men that call themselves Physicians betray their Anatomy, and their Philosophy, as well as their Divinity in their invectives against this Practice?"Eventually, inoculation did gain widespread acceptance.
The spread of print and increased literacy helped bring elite medicine to the masses through home medical manuals. Many ministers as well as their followers had copies of some of the most popular manuals, including William Buchan's Domestic Medicine (1769) and John Wesley's Primitive Physick (1747).
Wesley, better known as the founder of Methodism, was extremely interested in the democratization of medicine. Believing that medicine and doctors were often only accessible to the wealthy, he wrote his own medical manual to allow anyone to easily treat themselves. Wesley considered making medical knowledge and treatments available and comprehensible to the public was part and parcel of his pastoral duties.
The minister-doctor predominated until at least 1750 and continued on to the early 19th century in some areas of the country. It was only with the rise of medical education, apprenticeships, and licensing laws that the two forms of healing separated. But ministers continued to play an integral role in healing, particularly in many of the alternative health movements (homeopathy, hydropathy, osteopathy, etc) that flourished in the 19th century.