Virus- Organisms at the edge of life

The ongoing coronavirus outbreak has renewed a newfound interest in the virus species. They take their name from Latin ‘virus’ and Sanskrit ‘vish’, both meaning poison. The virus is so small that an electron microscope can only see it. They grow only inside the living cells of an organism. They infect anything that is living, including bacteria and other micro-organisms. They are the most numerous entities and we find them in all types of ecosystems ranging from boiling to freezing. They have probably existed since the first living cells evolved. Their origin is unclear as they do not leave fossils behind. Since the discovery of tobacco mosaic virus, we have discovered 5000 species. Some of them are present in the dark corners of the earth, waiting to unleash.

Some scientists consider them living because of the presence of genetic material. Others consider them non-living due to the lack of a cell structure. They don’t have their own metabolism and can make products only inside the host cells. They can spread in various ways through vectors, coughing and sneezing, faecal-oral route and food or water. HIV spreads through sexual contact. Common viruses prevalent in humans are Influenza, Norovirus, Rotavirus, rabies, smallpox, and HIV. We classify them according to the organisms they infect, as narrow range and broad range. Most viruses that infect plants do not harm animals, and most animal viruses do not harm humans.

They provoke an immune response which usually eliminates viruses from the body. But some viruses that cause AIDS and hepatitis evade the immune response of the body and result in chronic infections. The viruses continue to replicate in the body, bypassing the defence mechanism of the host. Such people are called carriers. If there is a high proportion of carriers, then we say that the disease has become an epidemic. Smallpox decimated native Americans population, brought by Europeans in the Americas, by as much as 70%. This aided the colonization efforts.

When an epidemic becomes worldwide, it becomes a pandemic. The 1918 flu pandemic killed nearly 5% of the world population. AIDS is a pandemic that claims millions of lives every year. We identified Ebola virus in 1976 and it has since spread intermittently with very high mortality rates. Viruses also cause cancers of various types. Since they are difficult to eliminate without drugs, toxic effects are seen in the host cells in general. The most effective approach that has been devised is vaccination and antiviral drugs, which selectively interfere with viral replication.

Viruses also have many applications. They are important means for transferring genes from one species to another. They have played a central role in the evolution process. Scientists use them as vectors to introduce genes, for making the cells produce a foreign substance and treating diseases. In nanotechnology we treat them as living Nano-particles. There is also concern that they can be used as biological weapons. We need to study viruses properly to eliminate the disease causing ones and apply the good viruses for human use.