When do human diseases infect international relations? Existing theories of cooperation emphasize the importance of repeated interactions. However, the COVID-19 pandemic demonstrates cooperation can break down during health crises. I address this discrepancy by developing a militarization theory to explain variation in patterns of health diplomacy. During periods of high security competition, militarization inhibits cooperation. During periods of low security competition, militarization enables cooperation. I operationalize this theory using a quantitative measure of security competition and qualitative framework for detecting militarization. Then, I investigate four cases of health diplomacy: the smallpox eradication effort, UNAIDS, the Global AIDS Initiative and PEPFAR, and the Global Health Security Agenda. All four cases support a militarization theory of cooperation. This consistent finding suggests elites pursue militarization strategies to achieve desired policy outcomes within the international health regime. Militarization theory has important implications for other international regimes, including nuclear non-proliferation and international trade.