Infectious diseases are a leading cause of death, accounting for a quarter to a third of all deaths worldwide. The spread of infectious diseases results from both human behavior such as lifestyle choices, land-use patterns, increased trade and travel, and inappropriate use of antibiotic drugs, as well as mutations in pathogens. These excerpts from a January 2000 National Intelligence Estimate highlight the rising global health threat of new and reemerging infectious diseases. The National Intelligence Council argues that the infectious disease threat will complicate U.S. and global security over the next twenty years. These diseases will endanger U.S. citizens at home and abroad, threaten U.S. armed forces deployed overseas, and exacerbate social and political instability in key countries and regions in which the United States has significant interests, according to the report.