Outbreaks of cholera, measles and yellow fever can spread rapidly and be fatal. Malaria is endemic in more than 100 countries. Millions are living with HIV/AIDS and tuberculosis. Viral haemorrhagic diseases such as Ebola or Marburg are rarer, but are potentially fatal. The coronavirus COVID-19 pandemic has affected virtually every country on earth, infecting tens of millions of people and resulting in the death of over a million more.

The people at highest risk are mainly those living in poverty or in areas of great instability, such as in armed conflict or during mass displacement, where living conditions are precarious, access to healthcare is limited, and vaccination coverage is low.