Almost all pandemics start with a single infection event. For zoonoses from wildlife, this is a person, or group of people that made contact with an animal infected by a pathogen that infects them, replicates in their cells and then is transmitted to others. Surveillance data suggest that spillover events happen frequently around the world, but most infections are unable to cause further transmission among people. Sometimes, pathogens spill over and are able to transmit to a handful of people, undergoing a few cycles of transmission before the outbreak dies out. Where pathogens spread into dense human communities (e.g. COVID-19 within the live animal market and city of Wuhan 10 ), and when they are able to easily transmit from person-to-person, they can become pandemics. Preventing pandemics will require efforts to reduce the risk each of these stages occurring, through measures that diminish the underlying drivers of spillover, their spread among people and their ability to move globally through rapidly urbanizing landscapes, megacities and travel and trade networks.
Pandemics represent an existential threat to the health and welfare of people across the planet, and their emergence, impact and control are deeply embedded in biodiversity and the major causes of biodiversity loss. New diseases emerge largely in tropical or subtropical countries with high wildlife biodiversity. The first people to be infected are often from communities in remote or rural regions, in developing countries with lower capacity to rapidly diagnose and treat novel diseases, and control and contain pandemic spread. Land use change and the wildlife trade (especially unsustainable, illegal or poorly regulated wildlife trade) are key drivers of pandemic emergence, including the recent emergence of COVID-19. Pandemics, such as COVID-19, underscore both the indivisible interconnectedness of the world community and the rising threat posed by global inequality to the health, wellbeing and security of all people: Exponential growth in consumption of products from land use change and globalized trade, often driven by developed countries, have led to the repeated emergence of diseases from developing countries with high biodiversity, and thus conditions that increase potential for zoonotic emergence.