Governmental health records and survey data. A lot of the really big stuff we use requires academic licenses, but there's still a lot of publicly accessible data.

For the U.S. try

- CDC's National Center for Health Statistics: http://www.cdc.gov/nchs/

- CDC WONDER: http://wonder.cdc.gov/

- NIH's Unified Medical Language System: http://www.nlm.nih.gov/research/umls/

And for global try the Global Health Data Exchange: http://ghdx.healthdata.org