Asbestos, lead paint, cigarettes, heroin(perscribed generously for basically whatever the doc felt like), "Radithor" (patent medicine containing radium-226 and 228, marketed as a "perpetual sunshine" energy tonic and cure for over 150 diseases), bloodletting, mercury treatments for syphilis, tobacco smoke enemas (yep that was a real thing), milk-based blood transfusions.
Didn't understand those either and used the fuck out of them because "the experts" said we should.