Lying somewhere between the living and non-living, the organic and inorganic, viruses have a strange propensity to kill their host cells. In fact, they cannot survive or multiply without a host, and their demise becomes only a matter of hours or days in its absence. An analogy may be drawn with the mythical monster that devoured its own master 'Frankenstein', as recounted in the famous novel by the 19th century English novelist Mary Wollstonecraft Shelley.
When compared with microscopic organisms that cause diseases in human bodies, viruses are the smallest of all in size, followed by larger bacteria and even bigger parasites. The word 'virus' is one of the most widely used term in present-day world mainly because of computers and the dread of computer viruses. Some people may fancy the existence of mental viruses that can wreak havoc inside human minds through psycho-somatic disorders.
The word virus comes from a Latin word that implies poisonous liquid. Viruses are genetic materials contained in organic particles that invade living cells and use the host's metabolic processes to produce new generation of particles. They are sub-microscopic infectious agents that replicate only inside living cells. They can infect all living beings including plants, animals, and micro-organisms like bacteria. They are found in almost all ecosystems on earth. Although the types of viruses in environment may be millions, over 5,000 viruses have been listed so far. The discipline that studies viruses is called 'Virology', which is a sub-discipline of 'Microbiology'. Sizes of viruses vary from the 17 nanometre 'Porcine Circovirus' to the 2.3 micrometre 'Tupanvirus'.
When not inside infected cells, viruses exist in the form of free particles or 'vitrions', consisting of genetic material or long molecules of DNA or DNA that encode structure of proteins by which the virus acts. A protein coat - the 'capsid' - surrounds and protects the genetic material. They can spread in numerous ways including transmission through disease-bearing organisms called 'vectors'. For instance, they may be transmitted among plants through insects; or blood-sucking insects may do the job in case of animals. Flu viruses are spread through coughing or sneezing, while 'norovirus' and 'rotavirus' through faecal-oral routes causing gastroenteritis. HIV viruses are transmitted through sexual contacts and infected bloods. Usually, viral infections in animals provoke immune response that can potentially eliminate the infecting viruses. When this response fails, the viruses win the battle.
The Novel Coronavirus or SARS-CoV-2 causes the disease Covid-19, which is atypical pneumonia - not flu. It belongs to a completely different family of viruses, and causes a disease having different symptoms and spreads.
The coronaviruses are of seven types. Of them, the earlier four have been pestering humans for over a century by causing a third of the common cold. The other two are tagged with 'MERS' (Middle-East Respiratory Syndrome) and 'SARS' (Severe Acute Respiratory Syndrome) that cause very severe diseases.
Why has this seventh coronavirus caused a global pandemic? To begin with, its structure provides some clue to its success. It is basically a spiky ball, and the spikes can recognise and stick to a protein (ACE2) that is found on the surface of human cells. This bonding is far stronger than SARS-classic, which becomes crucial during person to person transmission.
The newest coronavirus has two connected halves, and the spike activates when these halves get separated, thereby enabling the virus to enter cells. This separation occurred with some difficulty in case of SARS-classic, but the two halves of Novel Corona can be easily cut by an enzyme called 'furin' produced by human cells and found across tissues. Whereas many respiratory viruses tend to infect either the upper or lower airways, the Novel Corona appears to infect both, possibly because of its ability to exploit 'furin'. That probably explains why the virus can spread among people before the symptoms appear - an attribute that has made its timely control very difficult.
Despite its animal origin, the Novel Corona appears to be highly effective in infecting humans. Virologists believe, it has originated from a bat and then infected humans either directly or through another animal. A brief period of mutation was required by SARS-classic for recognising ACE2, but the new virus could do it from the very outset. It does not appear to have changed in important ways since the start of the pandemic. According to Lisa Gralinski of the University of North Carolina, "The virus has been remarkably stable given how much transmission we've seen. That makes sense, because there's no evolutionary pressure on the virus to transmit better. It's doing a great job of spreading around the world".
Researchers have provided a preliminary account of what the new virus does to the infected people. It attacks the ACE2-bearing cells that line the human airways after hitting the body. Dying cells then fill the airways with junk, carrying the virus deeper inside toward the lungs. The lungs get clogged with dead cells and fluid, thereby making breathing more difficult. The body's immune system then fights back and attacks the virus, resulting in inflammation and fever. But the immune system goes berserk in extreme cases and causes more damage than the actual virus. These damaging over-reactions are known as 'Cytokine storms', which were responsible for many deaths during the 1918 Spanish Flu pandemic, the 2003 SARS outbreak and the 2009 bird flu pandemic.
Now, the glimmer of hope for Bangladesh and her neighbours is that the coronaviruses tend to be winter viruses, and consequently the alarming trends are likely to be reversed in the heat and humidity of summer, as was the case with SARS-classic.
Dr. Helal Uddin Ahmed is a retired Additional Secretary and former Editor of Bangladesh Quarterly.
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