A good place to begin when looking for data on health and disease. Arranged by topic, or under a category for Healthy Living, which includes a variety of health related topics (smoking, obesity, nutrition, environmental health, etc).
Inter-University Consortium for Political and Social Research. ICPSR maintains a data archive of more than 500,000 files of research in the social sciences. It hosts 16 specialized collections of data in education, aging, criminal justice, substance abuse, terrorism, and other fields.
A collection that allow users to access statistics and population data interactively. Read the "Getting Started Quick Guide" before you do anything else. It is from the CDC, but may get you to the datasets quickly. Also, take note of the "Data Access" links on the left.