There was a lot going on. Germ theory, of course, was part of it. But public health measures, especially sewerage systems, clean drinking water, and municipal waste removal, were all massive contributors. Note that the decline in mortality occurs well in advance of antibiotics and even most vaccinations.

For all the recent debate on vaccinations, it's interesting to note that the peak period of their impace (roughly 1930 - 1960) saw relatively little reduction in mortality, though there was a large decrease in disease incidence. It turns out that with septic control, antibiotics, food quality, and nutrition, many viral diseases weren't killers, but did present quality-of-life issues. And yes, often quite severe -- polio was no joke, and I know people who've suffered lameness from it myself. Measles and smallpox are similarly scarring and have long-term impacts.

But the major impacts of virtually all medicine are front-loaded to the period before 1950, with much the gains since attributable to either greater access (especially for the disadvantaged) and removal of environmental agonists (lead, tobacco, alcohol, asbestos, miscellaneous poisons, safety hazards).