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.