The SARS-CoV-2 virus of the COVID-19 pandemic, that is presently devastating the entire world, had been active well before January of this year, when its pathogenic potential exploded full force in Wuhan. It had caused the onset of small disease outbreaks in China, and probably elsewhere as well, which failed to reach epidemic potential. The distant general origin of its zoonosis can be traced back to the ecosystem changes that have decreased biodiversity, greatly facilitating the contacts between humans and the animal reservoirs that carry pathogens, including SARS-CoV-2. These reservoirs are the bats. The transition between the limited outbreaks that had occurred through 2019 and the epidemic explosion of December–January was made possible by the great amplification of the general negative conditions that had caused the preceding small outbreaks. In the light of what we have now learned, the explosion was predictable, and could have happened wherever the conditions that had allowed it, could be duplicated. What could not have been predicted was the second transition, from epidemic to pandemic. Research has now revealed that the globalization of the infection appears to have been caused by a mutation in the spike protein of the SARS-CoV-2, that has dramatically increased its transmissibility.

The loss of biodiversity in the ecosystems has created the general conditions that have favored and, in fact, made possible, the insurgence of the COVID-19 pandemic. A lot of factors have contributed to it: deforestation, changes in forest habitats, poorly regulated agricultural surfaces, mismanaged urban growth. They have altered the composition of wildlife communities, greatly increased the contacts of humans with wildlife, and altered niches that harbor pathogens, increasing their chances to come in contact with humans. Among the wildlife, bats have adapted easily to anthropized environments such as houses, barns, cultivated fields, orchards, where they found the suitable ecosystem to prosper. Bats are major hosts for αCoV and βCoV: evolution has shaped their peculiar physiology and their immune system in a way that makes them resistant to viral pathogens that would instead successfully attack other species, including humans. In time, the coronaviruses that bats host as reservoirs have undergone recombination and other modifications that have increased their ability for inter-species transmission: one modification of particular importance has been the development of the ability to use ACE2 as a receptor in host cells. This particular development in CoVs has been responsible for the serious outbreaks in the last two decades, and for the present COVID-19 pandemic.