The National Institutes of Health (NIH) is a biomedical research facility primarily located in Bethesda, Maryland, USA. An agency of the United States Department of Health and Human Services, it is the primary agency of the United States government responsible for biomedical and health-related research. The NIH both conducts its own scientific research through its Intramural Research Program (IRP) and provides major biomedical research funding to non-NIH research facilities through its Extramural Research Program. With 1,200 principal investigators and more than 4,000 postdoctoral fellows in basic, translational, and clinical research, the IRP is the largest biomedical research institution on Earth, while, as of 2003, the extramural arm provided 28% of biomedical research funding spent annually in the US, or about US$26.4 billion.
The NIH comprises 27 separate institutes and centers that conduct research in different disciplines of biomedical science. The IRP is responsible for many scientific accomplishments, including the discovery of fluoride to prevent tooth decay, the use of lithium to manage bipolar disorder, and the creation of vaccines against hepatitis, Haemophilus influenzae (HIB) and human papillomavirus.