For instance, HIV was introduced in the human population from non-human primates in the 1920s or even before, and the AIDS pandemic emerged between the 1960s and 1990s, subsequently rapidly spreading throughout the world ( Gilbert et al., 2007 Faria et al., 2014 Worobey et al., 2016 Gryseels et al., 2020). In other words, these diseases originate from pathogens that were transmitted from non-human animals to humans, through a process known as zoonotic spillover. Regardless of the differences between these estimates, it is evident that most human infectious diseases have a zoonotic origin. Other estimates suggest that this percentage reaches up to 75% ( Taylor et al., 2001). Conservative estimates indicate that around 60% of all emerging human infectious diseases have a zoonotic origin ( Woolhouse and Gowtage-Sequeria, 2005 Jones et al., 2008). As a matter of fact, several pathogens that infect humans are derived from other animal species. Some of them take on pandemic proportions, reaching populations on different continents.
Outbreaks and epidemics are frequent events in human history.
Finally, this article discusses actions to prevent or reduce the frequency of zoonotic spillover events. As an example, the factors involved in the emergence of Severe Acute Respiratory Syndrome Coronavirus 2 (SARS-CoV-2) pandemic are discussed, indicating what can be learned from this public health emergency, and what can be applied to the Brazilian scenario. In this context, this article describes the basic aspects of zoonotic spillover and the main factors involved in spillover events, considering the role of the inter-species interactions, phylogenetic distance between host species, environmental drivers, and specific characteristics of the pathogens, animals, and humans. Understanding the factors that facilitate the transmission of pathogens from wild animals to humans is essential to establish strategies focused on the reduction of the frequency of spillover events. This demonstrates that spillover has a fundamental role in the emergence of new human infectious diseases. Most human infectious diseases (60-75%) are derived from pathogens that originally circulated in non-human animal species. The transmission of pathogens from wild animals to humans is called “zoonotic spillover”.
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