15. Invasion and Eradication

Pathogens invade new host niches all the time. The global invasion of the human niche by SARS-CoV-2 during the 2020–22 pandemic is the most recent example, but cross-species transmission is ubiquitous. In 2009 Influenza A/H1N1pdm09 emerged and spread globally most likely after a triple recombination of human, avian, and porcine viral segments (Smith et al., 2009a). The HIV-1 pandemic started in the mid-twentieth century probably from bushmeat spillover of chimpanzee simian immunodeficiency virus, which itself is thought to have originated from spillovers from other primates, to go global in the 1970s (Hemelaar, 2012). Cross-species transmission is not just an issue of zoonotic spillover or anthropogenic spillback, it is equally important as spillover among animal species.