Where Do Viruses Come From And What Makes Them So Deadly?
Viruses can be described as the “organisms at the edge of life” - between living and non-living beings.
The recent Covid-19 pandemic has infected 180,000 people worldwide and killed over 7,000 people.
In this two-part series, we shall explain how viruses work, and how an ancient civilisation like India, that has evolved in a tropical climate, imbibes lifestyles and values to deal with the overarching existential threat of epidemics.
What is a virus?
Viruses are small biological entities with a diameter of less than 200 nanometers (nm), about one hundredth the size of most bacteria.
They can only replicate within a host. Once in the host cell, the viruses can coax it into making new copies of the virus.
When outside a host, a virus exists as an independent particle which consists of
- the genetic material - DNA or RNA that encode the structure of the proteins by which the virus acts
- a protein coat, the capsid, which surrounds and protects the genetic material
Viruses do not carry out metabolic processes, and unlike living organisms, they cannot generate ATP.
No known virus contains ribosomes for translation, that is, a virus cannot synthesise proteins to perform a specific cell function (the process of gene expression).
Therefore, virus are described as the "organisms at the edge of life" - between living and non-living beings.
Without the host cell, a virus cannot survive long term.
A recent study determined that Covid-19 could survive up to three days on stainless steel and plastic surfaces, just one day on cardboard and four hours on copper. The results indicated that novel coronavirus can live in the air for hours and on surfaces up to days.
How did viruses evolve?
There are three main hypotheses regarding this.
The progressive hypothesis states that viruses arose from genetic elements that gained the ability to move between cells.
The regressive hypothesis asserts that viruses are remnants of cellular organisms that adapted themselves to a parasitic replication strategy.
The virus-first hypothesis states that viruses predate or co-evolved with their current cellular hosts, that they are the link between the living and the non-living world.
Today, viruses are the most abundant biological entities. Every organism in existence, from a tiny bacteria to large blue whales, has viral parasites.
There are more bacteria in and on all the cells in our bodies. But there are ten times as many viruses. If alive, viruses would have been the dominant organisms of our times.
What is coronavirus?
There are several different types of viruses.
Coronavirus is one such family of viruses that causes illnesses ranging from the common cold to more severe diseases such as Middle East Respiratory Syndrome (MERS) and Severe Acute Respiratory Syndrome (SARS).
Coronaviruses are zoonotic, that is, they are transmitted from animals to people.
The investigations have revealed that SARS was transmitted from civet cats to humans and MERS from dromedary camels to humans.
Major modern diseases such as Ebola and salmonellosis are zoonotic. HIV was a zoonotic disease transmitted to humans in the early part of the 20th century, though it has now mutated to become a separate human-only disease.
In fact, out of the 1,415 pathogens known to infect humans, 61 per cent are zoonotic.
There is increasing genetic evidence that most human diseases, including measles, smallpox, influenza, HIV, and diphtheria , various forms of the common cold and tuberculosis, originated in other animals.
To infect a particular kind of host, the viruses must evolve themselves continuously.
Covid-19 is a strain of the coronavirus that has recently evolved itself to affect humans.
Many different strains of coronavirus are circulating among different animals which do not affect humans. This is because they haven’t yet genetically evolved to colonise the human body.
How to protect oneself from viruses?
Viruses evolve in tandem with their hosts and the vagaries of their behaviour. In response, the host animals also evolve to protect themselves against infection. However, Viruses evolve quicker.
Billions and billions of viruses around us exist in an asymptomatic balance based on constant struggle. However, due to genetic evolution, this balance can be broken, and that results in the events like the current Covid-19 pandemic.
It’s believed Covid-19 infection started in a “wet market” in Wuhan, China, where farmed and exotic animals are tied up or stacked in cages, and often killed on-site to ensure freshness.
The virus is closely related to known bat viruses. That’s why it’s believed to have originated from a bat. However, since bats were not sold at the Wuhan market, it’s likely that there was an intermediate host.
So, there are two levels of protections that humans can have.
One is short term - to protect oneself from coming in the contact of a virus that can cause disease. For example, washing hands, taking bath, social distancing, etc.
Second is long term - to provide as less opportunity as possible to the viruses to adapt themselves to the human host.
In Part II, we shall turn to the interesting interplay of nature and culture in the Indian civilisation, and how Indians dealt with the existential threat of epidemics.
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