If a new disease treatment is created that is far cheaper and more effective than previous treatments, GDP would fall but consumer surplus would rise (medical expenses portion of basic cost of living would decrease).
Why would GDP fall? Unless you think the consumer will not spend the money (i.e. my insulun use to cost $200 per month but now I have this pill for $20 per month which does the same thing so I guess I should burn this $180 left over)?
Because the savings rate has been negative in this country since around 2001 so that $180 left over will go to paying down debt, which causes deflationary pressure.