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The CDC's main mission is "to protect America from health, safety and security threats, both foreign and in the U.S." To that end, it provides the most current, reliable information on diseases affecting the American people, whether it be through current outbreaks or ongoing exposures/lifestyle choices.
The NIH mission statement is "to seek fundamental knowledge about the nature and behavior of living systems and the application of that knowledge to enhance health, lengthen life, and reduce illness and disability." The website provides news articles on health and medical developments as well as links to some free medical library owned publications.