Death rates decreased because symptoms and complications were able to be treated better over time eg with hospitalisation, antibiotics for complications etc, not because the diseases became less prevalent.
In societal terms, it could be argued that disability from disease is more harmful to society than immediate death. Certainly it's more costly. The cost of lifelong support for someone with disabilities and recurring complications from polio for example, or those blinded by or with reduced mental capacity from measles etc.
