Emerging microbial pathogens were responsible for the large epidemics of the past, and they continue causing outbreaks, epidemics, and pandemics around the world. Several infectious agents, such as dengue, chikungunya, malaria, and tuberculosis, are endemic in Latin America, negatively impacting public health in several countries in the region (Yeh et al. 2021). The microbial diversity in Latin America (Calisher 2013) is responsible for the continuous emergence of novel human pathogens.
Emerging viruses can cause self-limiting outbreaks when they infect isolated populations. However, in an increasingly interconnected world, these outbreaks can progress to regional epidemics, or even to pandemics, as we recently experienced with SARS-CoV-2 and COVID-19.
Historically, epidemics have been a constant scourge of humanity since the time when the first cities began to be established around the year 7500 B.C. in the fertile valleys of Mesopotamia, on the banks of the Nile River, and in the valleys of the Indus and Yellow rivers in Asia, among others. New pathogens, especially those coming from animals (zoonoses), managed to establish themselves in these populations, causing epidemics when they reached the critical mass needed to maintain the circulation of the new pathogen without depleting the population of susceptible individuals. It has been estimated that the minimum population density needed to sustain epidemics varied according to the pathogen, although it’s thought to be in the order of 200,000 people (Dobson and Carter 1966).
Pathogens that are newly introduced among humans find populations without previous immunity and can cause widespread epidemics and pandemics. These pathogens establish a balance between their pathogenicity and the possibility to continuously circulate endemically in the human population. In the past, when cities grew in number and size, and communications were established between them, pathogens spread more widely and led to the first major epidemics and pandemics (McNeill 1976). With the increase in the world’s population and its exposure to new pathogens, the danger of new epidemics and pandemics is real (Morens et al. 2004, 2008; Morens and Fauci 2013; Esparza and Vizcaino 2021).
