What is a Virus? What are Viruses made up of ?

6. Why do some viruses only affect certain host species?

Several viruses have a narrow host range and will, for instance, only infect certain animals. For example: Human papilloma virus (HPV); Human Immunodeficiency Virus (HIV); or measles virus only infect humans. Viruses, in general, do not achieve anything by killing their host, as they can no longer replicate if their host has died. Therefore, many viruses have co-evolved with their hosts and do not cause severe disease. [Ref: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3411105/.]

Sometimes, a virus can jump the species barrier and infect a host that it doesn’t usually infect. This often results in a more severe disease as the virus is not adapted to this host and/or the host immune system will not have previously encountered the new virus. For example, although some types of influenza A virus typically replicate in birds, humans or pigs, occasionally, a “mutant” avian (bird), or a pig (swine) influenza virus emerges that is able to infect humans. Most mutants are “less-competent” viruses, unable to transmit or replicate well in host cells, and only very occasionally does one arise that is capable of efficient transmission and replication in humans. That’s why these species-jumping events are carefully monitored as they can lead to the evolution of new dangerous viruses that can cause pandemics. Many are famous: Spanish flu caused over 50 million deaths in 1918; In 1957, Asian ‘flu caused > 1 million deaths; H1N1 “swine” ‘flu infected 60 million people in 2009, but only caused 12,000 deaths. When a highly infectious virus jumps the species barrier, there is no pre-existing immunity in the new host population, so it can potentially have a major impact. The implications of species-jumps in the context of the origins of SARS-CoV-2 will be discussed later.