Scientists have estimated that there are 10 nonillion (10 to the 31st power) viruses currently on our planet. They are everywhere. Many viruses are beneficial for their host, many inflict no harm, but why do so few viruses affect us and even fewer severely affect us? The short answer: “These pathogens are extraordinarily picky about the cells they infect, and only an infinitesimally small fraction of the viruses that surround us actually pose any threat to humans” says virologist Sara Sawyer.
Understanding how certain viruses affect humans is crucial for protecting and preventing future outbreaks. COVID-19, the most recent outbreak that experienced a “spillover event,” was initially spread through interactions with an animal that is a “non-human primate”. This is called zoonosis. Multiple outbreaks have been introduced this way, but not can be started this way. Pathogens can also enter through cuts, scrapes, mosquitoes, ticks, etc. Once a virus has entered, it needs to find a way to get inside the cells and replicate. To do this, it must first attach to the surface of a host cell and then inject its genetic material (RNA) into the cell. The virus’s genetic material then takes over the machinery of the host cell, using it to replicate itself and produce new viruses. Viruses with a lot of genetic flexibility, and particularly those that encode their genomes as RNA rather than DNA, are well-suited to crossing the species divide. The majority of pathogens that have infected the human population in recent decades have been RNA viruses, including Ebola, SARS, MERS, Zika, several influenza viruses, and SARS-CoV-2. The more lethal viruses were found to have been hiding in their hosts for longer periods of time before showing any symptoms. This would allow it to replicate and spread to new species.
So the answer is; that a virus has to be incredibly sophisticated for it to cause harm to a human, pandemics are so rare because of precautionary measures such as vaccines, healthcare, and proper sanitation. The continuous study of viruses and their behavior is an important task for the human population and its future as current viruses are continuously mutating and developing with each given day.