Why do only some viruses infect humans?

Investigating how viruses cross the species-divide

Viruses; there are more of them than there are stars in the universe. They are highly infectious and often dangerous. Generally considered to be non-living, viruses are pathogens capable of infecting a host and then multiplying, having a negative impact on the host. Viruses exist everywhere, from the water we drink, to the air we breathe, to the ground we walk. From the number of viruses with which humans co-exist, it would make sense for our immune systems to be battling several of them continuously. However, most viruses do not impact humans (though there is still a large number that do). This is not because humans have impenetrable immune systems, but to do with the viruses themselves. Viruses cannot infect all living organisms. There are viruses such as Avian Influenza (bird flu) which only affects birds such as ducks and chickens, and cannot transfer to humans, and so are no threat to them. However, sometimes a virus crosses the species-divide; as was the case with COVID-19, a virus which started out as affecting bats, but managed to adapt to also infect humans.  

To cross the species-divide, a virus must cross several hurdles. It must first establish one infection in its new host by moving from one organism to a completely new one. This is incredibly challenging, because the receptors on the surfaces of cells are different in different organisms, and if the virus cannot latch onto the receptor, it cannot establish a new infection.  The process is a little like a key in a lock; if the key (the virus) is not complementary to the lock (the receptor), it cannot fit into it and infect the cell. SARS-CoV-2 (the coronavirus that causes COVID-19) engages with the protein ACE2 to enter cells in the human airway. After the virus has established itself in the first host, it must now infect others. However, this is where a lot of the viruses that have attempted to make a leap stop. They cannot progress past the first individual and so the individual becomes a dead end. A virus is more likely to progress to and past the first individual if they have more genetic flexibility. Viruses that encode their genomes with RNA rather than DNA are more likely to have greater genetic flexibility. This is because when viruses made with RNA copy their genetic code from one organism to another, they tend to be more imprecise. This creates mutations, which lead to more variety. The variety means the replicated virus may be more likely to survive in different environments, or enter new molecules in different hosts. Viruses such as Ebola, the Zika virus and SARS-CoV-2 all code their genomes with RNA.  

The way viruses move between species is complex, but by learning more about the process, scientists can monitor viruses and may be able to predict and therefore prepare for or minimise potential threats that can arise from them crossing the species-divide.  

Anika V