What is a virus?

A virus is essentially a parasite that cannot reproduce by itself. It needs a susceptible host cell for it to replicate. An infectious virus particle called virion consists of just nucleic acid (either DNA or RNA) and protein. Once a virus infects a host cell it will then direct the host cell machinery to produce more viruses.

Coronaviruses are positive-stranded RNA viruses with a crown-like appearance under an electron microscope (coronam is the Latin term for crown) due to the presence of spike glycoproteins on the envelope. 

COVID-19

In the last two decades, three coronaviruses have crossed the species barrier to cause deadly epidemics. The severe acute respiratory syndrome coronavirus (SARS-CoV) in 2002 to 2003, and H1N1 influenza in 2009 and Middle East respiratory syndrome coronavirus (MERS-CoV) in Saudi Arabia in 2012 were recorded.

The latest one and the most deadliest yet is the current pandemic caused by the new coronavirus. COVID-19 is an acronym for coronavirus disease 2019. Initially, the new virus was called 2019-nCoV. Subsequently, the task of experts of the International Committee on Taxonomy of Viruses (ICTV) termed it the SARS-CoV-2 virus as it is very similar to the one that caused the previous SARS outbreak (SARS-CoVs).

Transmission of a virus/ disease from animals to humans is called zoonotic transmission. Both SARS-CoV and SARS-CoV-2 are closely related and originated in bats, which most likely serve as reservoir host for these two viruses. Palm civets and raccoon dogs have been recognized as intermediate hosts of SARS-CoV between bats and humans. The SARS-CoV-2 intermediate host remains unknown although some studies suggest it is the pangolins.

Why do the viruses jump from animals to humans?

Viruses by themselves do not reproduce. First they replicate in a primary host which are mostly mammals or birds which are called the reservoir host. Then when another species like some wild animal comes in contact with the reservoir they contract the virus and that is called exposure. In the next stage called breakthrough, the virus overcomes the immune response of the new host, and transmission happens. The virus can now spread from one host to another, like humans.

Virus (COVID 19)–> Reservoir Host (like bats)–> Intermediate Host (pangolins?)–> Humans

When does zoonotic transmission become problematic in humans?

We are exposed to animals through the food/meat that we eat, the pets that we keep, and our interactions with nature. Luckily, majority of animal viruses are harmless and they pass through our gastrointestinal tracts or are destroyed by our immune systems. <0.1% of all the animal viruses cause infection in humans.

Most animal viruses also do not replicate in humans because they do not have all the machinery required to do so. Secondly, the virus will try to adapt and improve itself, by mutating and evolving itself in the human environment which can make it more potent.

The most dangerous animal viruses are the ones which require very few mutations to replicate in humans. They are probably already mutated in the reservoir and intermediate host and when see a human body they are already potent.

When an animal virus infects a human host and begins to replicate itself, executing its entire lifecycle within human cells and expanding one virion into a population of many. This causes a rapid increase in the viral titre in the human body which makes it easy to spread to another human and so on.

The spread of a viruses among a population is a complex process and depends on a number of factors like how well the virus is adapted to infect and multiply in the humans, the population density of a place, host genetics etc.

How are humans immune to animal viruses?

Human body has various defense mechanisms against pathogens like viruses. Our skin, mucosal layer and various acids in the alimentary canal stop the viruses at the port of entry. Even if the virus overcomes these barriers next comes our immune system. Our immune system has many many weapons to fight and overcome the infection. A dangerous virus unfortunately overcomes all these defense mechanisms to cause disease in humans.

Take Home: Dangerous animal viruses are those that require few mutations in order to begin replicating themselves in human cells. This means that these viruses have already attained the required combination of mutations most likely in their natural reservoir. To identify these dangerous viruses, we must identify the factors in the human immune system and body that are currently protecting us from each class of animal virus with high zoonotic potential and determine whether those are few or many.