Emerging Pandemic Diseases: How We Got to COVID-19

Abstract

Infectious diseases prevalent in humans and animals are caused by pathogens that once emerged from other animal hosts. In addition to these established infections, new infectious diseases periodically emerge. In extreme cases they may cause pandemics such as COVID-19; in other cases, dead-end infections or smaller epidemics result. Established diseases may also re-emerge, for example by extending geographically or by becoming more transmissible or more pathogenic. Disease emergence reflects dynamic balances and imbalances, within complex globally distributed ecosystems comprising humans, animals, pathogens, and the environment. Understanding these variables is a necessary step in controlling future devastating disease emergences.

Keywords: COVID-19; One Health; disease ecology; epidemic; epizootic; infection; medical history; pandemic; virology; zoonosis.

Figure 1

Global Daily Incident Cases of COVID-19 by World Health Organization Region as of August 18, 2020 The data (World Health Organization) show that beginning in March 2020, the pandemic exploded in Europe and the Americas, particularly in the United States, was blunted in these two regions between March and May 2020, and then began to explode anew in the Americas and to a lesser extent in Europe beginning in late May. Since May 2020, the pandemic has been increasing significantly in the SEARO as well as the AFRO regions. WRPO, Western Pacific; AFRO, Africa; EMRO, Eastern Mediterranean; SEARO, Southeast Asia; EURO, Europe; AMRO, Americas.

Figure 2

Recent Emerging Infectious Diseases The global extent of newly emerging, re-emerging, and “deliberately emerging” infectious disease from 1981 to the present (2020).

Figure 3

Infectious Agents, Hosts, and the Environment: Determinants of Disease Emergence and Persistence Diseases, including emerging diseases, result from interactions between infectious agents, hosts, and the environment. Adapted from Fauci and Morens (2012); Morens and Fauci (2012); Morens et al. (2004), (2008a).

Figure 4

Variable Mechanisms of Viral Entry into Host Cells Examples of cell receptors for various DNA and RNA viruses. The cartoon image shows a spherical cell with different receptors for different categories of selected viruses. Viruses and cells are not reflective of relative sizes. The figure is suggested by the text and images of Jayawardena et al. (2020).

Figure 5

Proposed Molecular Mechanisms of Host-Switching Proposed mechanisms of cross-species host-switching of infectious agents (after Kuiken et al., 2006). Steep (A) and shallow (B) fitness valleys between donor and recipient host species reflect adaptational barriers that need to be crossed. (A) and (C) show that a greater number of sequential adaptational mutations are needed in (A) to cross the fitness valley and then adapt to the new host, as compared to the situation shown in (B) and (D), where greater donor-host similarities facilitate switching. (C) and (D) represent the associated phylogenetic trees: in (C), the donor host and recipient host viruses most go through significant adaptational steps, including those associated with initial transmission in the new host. In (D), the new host receives an infectious agent that is partially pre-adapted; successful emergence requires fewer adaptational mutations.