Abstract. The article focuses on the spread of mass diseases as one of the critical modern global challenges. It analyzes the negative consequences of this phenomenon, including medical, economic, social, political, and legal consequences. The author provides a brief overview of major epidemics that occurred in the recent history, assesses economic damage inflicted by them and identifies spheres of human activity most vulnerable to pandemics. The article also raises the issue of legal responsibility of states and international organizations for actions (or inaction) that contribute to the spread of mass diseases. Recommended strategies to increase the level of global epidemiological security include the need to both strengthen national health systems and ensure international exchange of knowledge, technologies and materials, including viruses and laboratory samples, and to develop and expand overall international cooperation in this field. The author argues that the World Health Organization should continue to play the key role in this process on the basis of its systems of epidemiological surveillance, global warning and response, its long and solid experience as a global leader in public health and its partnerships with governments, UN system organizations, civil society, academia, private sector and media.