MedlinePlusWebsite by the National Library of Medicine, provides data from many authoritative websites, including the various National Institutes of Health (NIH) sites, the American Cancer Society, and others.
A.D.A.M. Medical Encyclopedia (MedlinePlus) - Good source of images for illustrating your project.
Check the "List of NIH Institutes, Centers, and Offices" to find the NIH site relevant to your topic.
For example:
diabetes = NIDDK (National Institute of Diabetes and Digestive and Kidney Diseases)
COPD = NHLBI (National Heart, Lung, and Blood Institute)
cancer = NCI (National Cancer Institute)
infectious diseases = CDC (Centers for Disease Control and Prevention)
