We live and prosper in a cloud of viruses, but the frequent emergence of viral diseases such as the flu and now COVID-19 has lead to the vilification of these simple structures. I, a biologist, couldn't have told you exactly what a virus is before researching this article because virology was never a core topic of my degree. Considering that they outnumber cellular life ten-to-one and drive biological cycles globally, it seems inconceivable that we know and learn so little about them. Here is a short introduction to viruses and the emergence of viral diseases.
H5N1 strain of avian influenza, or bird flu.
Simply put, a virus is genetic material encased in a protein shell, bearing little resemblance to a cell. The smallest virus discovered so far only has three genes, compared to the few thousand genes of the average bacterium. The consensus among microbiologists is that viruses are non-living and exist in a grey zone between life-forms and chemicals, although some have argued that having infected a cell, a virus acquires the properties of what we define as life. A virus particle by itself is biologically inert, lacking the cellular machinery to reproduce and carry out metabolic activities. In order to replicate itself and pass on its genes, a virus must infect a host cell by binding to it and injecting its genetic material. Specificity to certain species and sometimes to particular tissues has evolved along with a 'lock-and-key' system by which a host will let a virus in. Once inside a cell, the virus essentially takes control of protein manufacture and DNA replication to produce identical copies of itself. Hundreds of thousands of new virus particles subsequently burst out of the cell and go on to infect additional cells. The symptoms we associate with viral infections, such as coughs and fever, do not actually result from this cell death, but from our bodies' defence mechanisms.
There are three main ways in which new viruses emerge and give rise to harmful diseases, the first one being the mutation of existing viruses. One example is the frequent mutation of the influenza virus, which each time becomes different enough to the ancestral virus that it bypasses the acquired immune defences. Another way for viruses to spread is through increased connectivity between human populations, with rare cases spreading globally. AIDS, for instance, seemingly appeared out of nowhere in San Francisco in the early 1980s, but had in fact been around and gone unnoticed since 1959 in the Belgian Congo. Finally, and most relevant to the current pandemic, viruses spread from animals to humans as we increasingly come into contact with wildlife through habitat destruction. An estimated three quarters of new viral diseases originate in our proximity to animals, especially livestock. Pigs and birds act as natural reservoirs for multiple viral strains of flu, which mix and recombine into new strains. Further altered by mutations, it is easy to see how such strains, once leaked to human hosts, give rise to unknown diseases with no immune defence barriers.
Our continued encroachment on the natural world will increase the frequency of viral disease epidemics globally. Climate change threatens to expand the range of disease-bearing animals and insects northwards, allowing viruses to jump from one host to another more readily. Mosquitoes carrying Dengue fever, for example, have migrated from their usual tropical habitats to Florida and Portugal, where cases of infection have since been reported. COVID-19 is not an isolated case to be forgotten the moment a vaccine is developed and the number of cases subsides. Somewhere, the next pandemic is brewing and we cannot afford to discount the risk and underfund projects that will prevent it.
