ISHAN KUKRETI
East Respiratory Syndrome (MERS). Despite high fatality rates—9.5 per cent in case the of SARS and 34 per cent for MERS—
none of these coronaviruses managed to cause large-scale outbreaks. While SARSCoV appears to have disappeared in 2004, MERS-CoV causes limited outbreaks. Now, call it a third time lucky or the outcome of an evolutionary strategy, SARS-CoV-2,
despite a low fatality rate of 2-5 per cent, has emerged as the most devastating pandemic since the 1918 Spanish flu.
The success of SARS-CoV-2 is no mean achievement, when compared with other thousands of pathogens that naturally get transmitted between animals and humans but more often than not fail to establish a disease in human populations, let alone cause epidemics (see 'Pathogen load').
Most of these zoonotic pathogens, be it a virus, bacterium, fungus or parasites (protozoa and helminths), are believed to be host-specific. This means they usually restrict themselves to a limited number of species, such as bats, pigs, rats and chimpanzees, and prefer residing in them by creating a life cycle reservoir. This trait of pathogens is due to species barriers. Along with the human body’s resilience system against diseases, species barriers help us most of the times lead a life free from infections, despite the fact that we live in a pathogen-filled world. Crossing it is not easy as these barriers are determined by the level of human exposure to pathogens—directly through faeces or body fluids like saliva, blood and urine, of an infected animal, or indirectly through areas where they live and roam, or contaminated surfaces—and the ability of pathogens to infect a human and cope with the new host’s immune response. It thus requires the pathogen to undergo specific changes through mutation or genetic exchanges with the host.
However, these mutations are not always successful. Thus, a vast majority of animal-to-human spillover most likely results in a dead-end for the virus (and other pathogens), says Abi Tamim Vanak,
ANTHRAX
New SARSCoV-2