The results of federally funded science research: "more than 200 million pages of authoritative U.S. government science information including research and development results.” Will include articles from PubMed and data sources from the CDC among its results.
Agencies of the Department of Health & Human Services
"The world’s largest biomedical library, NLM maintains and makes available a vast print collection and produces electronic information resources on a wide range of topics.” NLM databases include PubMed and MeSH.
MedlinePlus, as opposed to MEDLINE, is the NIH's database of health information for patients. MedlinePlus covers treatments, medications and supplements, medical terminology, and links to recent research on medical topics.
"Government information by topic and agency,” for federal, state, and local government data. The best place to start looking for a topic if you're not sure which agency/agencies would be involved, because it will bring back results from all of them! Everything else included in this list will generally also be findable via USA.gov.