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The spectrum of human pathogens and the infectious diseases they cause is continuously changing through evolution, selection and changes in the way human populations interact with their environment and each other. New human pathogens often emerge or re-emerge from an animal reservoir, emphasizing the central role that non-human reservoirs play in human infectious diseases. The 1918 pandemic of influenza virus A/H1N1 and the 2020 pandemic of coronavirus disease 2019 (COVID-19) are the most dramatic examples of this in recent human history. Pathogens can also re-emerge with new characteristics, such as multidrug resistance, or in different places, such as Ebola virus in West Africa in 2013 and Zika virus in Brazil in 2015, to cause new epidemics. Most human pathogens have a history of evolution in which they first emerge and cause epidemics, become unstably adapted, re-emerge periodically and then - without intervention - eventually become endemic, with the potential for future outbreaks.

Original publication




Journal article


Medicine (Abingdon, England : UK ed.)

Publication Date



is an Honorary Consultant in Clinical Microbiology with the University of Oxford, UK, and Director of the Oxford University Clinical Research Unit in Hanoi, Vietnam. Competing interests: I have received funding to attend and speak at a symposium on influenza vaccination and antimicrobial resistance organized by Sanofi (2019), and research funding to work on antimicrobial stewardship implementation from Pfizer (2018, Independent Grants for learning and Change, managed by the Joint Commission). I am a consultant for Wellcome on the board of the Surveillance and Epidemiology of Drug Resistant Infections Consortium (SEDRIC).