From the plague to Zika, from cholera to dengue fever, what are these epidemics that sometimes spread beyond borders to become pandemics?

The plague, Zika, cholera, dengue fever, AIDS, Ebola, and now COVID-19 are infectious diseases that humanity has to deal with on a regular basis. But how does a pandemic start? Well, it can often be difficult to identify the exact source at the outset. Generally, it is caused by a microbe found in nature, in the meat we eat, or on animals such as rats or fleas. As soon as the virus is transmitted to humans, the race against time begins. A pandemic is, above all, an epidemic that has spread worldwide. It is therefore necessary to act as quickly as possible to limit its spread.