To date, the Haitian cholera epidemic that broke out in 2010 has killed more than 8,500 people, and sickened another 600,000. Although United Nations (UN) peacekeeping forces have been widely blamed for introducing the bacterial pathogen into Haiti, the UN continues to deny responsibility and rejects demands for victim compensation. Recently, two human rights groups filed a class action lawsuit against the UN in federal court, seeking compensation for cholera victims. The suit, which ventures into largely uncharted waters of international law, takes place against a backdrop of intense and sometimes rancorous scientific debate about the human and environmental determinants of the epidemic. The UN is relying on a two-pronged defensive strategy: first, a defense based on immunity derived from its traditional diplomatic privileges and immunities, which dates back to the organization’s founding in 1946. Second, a defense based on a lack of proximate cause, which is bolstered by several prominent scientists’ theory that the pathogen may have been endemic to Haiti and only was unleashed by the combined effects of climate change, a devastating earthquake, and unusually violent weather episodes. In this article, I will discuss this evolving dimension of international law, particularly as it is intertwined with ongoing scientific and environmental controversy.