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Fig. 1.  Hybrid animal-human lifecycle of flu virus (

Viruses have
Multiple Hosts

Viruses can thrive inside any living cells as long as they can hijack the cell's replication machinery. They even infect bacteria and in rare cases other larger viruses, However, since animals have differences in their biological molecules including the polymerases and ribosomes, not all viruses can survive inside all cells. Instead, they have a group of hosts. Flu virus thrives primarily in birds, pigs and humans (Figure 1). Similarly, HIV (human immunodeficiency virus) has a common host among humans and apes. In apes it is called SIV (simian immunodeficiency virus). Simian as a synonym for apes. To differentiate from human viruses, the animal borne viruses are collectively called zoonotic viruses. There are a few types of plant viruses also. There are no known common hosts between plants and animals. The most likely reason is the vast difference in the biochemistry of plants and animals.

Antigenic Drift
and Shift

Viruses mutate constantly. Some mutations are driven by the evolutionary pressure to survive the host immune system while others are due to the accumulated errors during replication. Both of these naturally occurring mutations are called antigenic drift. However, sometimes catastrophic events occur. These happen most often when there is an overlap between humans and animals. In case of the 2009 Swine Flu, the flu viruses from poultry and humans got into the same cells in pig (Figure 1). As we learnt in the Section Biochemistry of Viruses, the viruses are split open inside the cell. The RNA from bird and human viruses intermixed inside the porcine cells. It was exacerbated by the open-ended segments of the flu virus RNA. That created a radically new flu virus. This process is called antigenic shift. The shift during the 2009 Swine Flu invalidated the current vaccine almost overnight. Since our immune system takes a long time to adapt, it cannot fight the antigenic shifts. It eventually catches up but that can take years. Mercifully, the reformulated vaccines expedite that process. The new vaccine in 2009 was so different that there was an unprecedented change of name, from A/H1N1 to A/H1N1pdm. The suffix is a shortened word for pandemic.


Fig. 2.  Coronavirus hosts (

Coronavirus and COVID-19

Coronavirus (COV) was first documented in humans in 1931. It remained quiescent until two novel strains were discovered in 1967. It was on the backburners of the researchers until the turn of this century when SARS-COV (2003), HCOV-HKU1 (2005), MERS-COV (2012) and SARS-COV-2/COVID-19 (2019) events occurred. Of these, only COVID-19 has been categorized as a pandemic. A pandemic is global outbreak while epidemic is regional. All coronavirus outbreaks had an overlap among animals and humans. It is believed that MERS was caused by the viruses moving between camels and humans. Bats are the suspected cause of COVID-19 (Figure 2).

Eradication of

it is true that viruses such as smallpox and polio have been effectively eradicated. Eradication depends upon herd immunity. If sufficient people are vaccinated and herd immunity is achieved, the virus runs out of the critical number needed by it to thrive. That is the impetus for the current drive to mass immunize against COVID-19. Eradication does not mean that smallpox and polio have vanished from Earth. Instead, their numbers have been reduced to an extent that they cannot mount an effective attack. Hopefully, the status quo will be maintained and those viruses will never stage a comeback. Smallpox and polio are rare successes because they do not have an animal host. We cannot get rid of flu virus until we either kill or vaccinate a sufficient number of water fowl. That is not possible. We cannot control the animal hosts of coronavirus either. Therefore, flu and coronavirus will be with us forever.

  • Liu, P. (2020). Are pangolins the intermediate host of the 2019 novel coronavirus (SARS-CoV-2)? PLOS Pathogens.

  • Long, J et al. (2019). Host and viral determinants of influenza A virus species specificity. Nature Reviews Microbiology. 17, 67-81(2019) Report.

  • Yuan, S et al. (2020). Analysis of Possible Intermediate Hosts of the New Coronavirus SARS-CoV-2. Frontiers in Veterinary Science. 7(379) e01031-17. DOI: 10.3389/fvets.2020.00379.


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