The Department of Defense (DoD) Global Emerging Infections Surveillance and Response System (DoD-GEIS) identifies and addresses DoD vulnerabilities to emerging infections through a global network of partners. Following the Indian Ocean tsunami of December 26, 2004, DoD-GEIS facilitated the DoD medical response and coordination with the Centers for Disease Control and Prevention and the World Health Organization. DoD-GEIS partners in Southeast Asia (U.S. Naval Medical Research Unit 2, Jakarta, Indonesia; and Armed Forces Research Institute of Medical Sciences, Bangkok, Thailand) rapidly conducted health assessments and established surveillance for communicable diseases that threatened survivors. Preexisting collaboration with the Centers for Disease Control and Prevention, the World Health Organization, and host countries was critical for the DoD-GEIS tsunami response.