Smallpox. Diptheria. Cholera. Plague. Dysentery. Typhus. Typhoid. Scarlet fever. Tetanus. Pertussis. Syphilis.
I think plumbers and engineers had as much to do with wiping out a few of those diseases as doctors did.
few does not equal none.
The fact that sewage lines, septic tanks and such are closed, underground, and 100+ feet away from wells/water lines, rather than open, germ-infested ditches right next to wells, vegetable gardens, etc. Also helps to keep the livestock in their own barn, detached from the house...