A virus is a non-living, submicroscopic pathogen that infects living host cells in order to replicate. A single virus “individual” is commonly referred to as a “virus particle”. All types of living organisms can be infected by viruses, including plants, animals, fungi, bacteria, and archaea. Viruses attack living cells and inject their genetic material (either DNA or RNA) into the cell. The “host” cells then create proteins based on the information in the genetic material that the virus injected. Those proteins work together to produce new virus particles. Viruses have evolved to use the energy and existing replication machinery or their host cells.

Data from virus genetics and viral morphology seems to indicate that various virus types likely evolved independently more than once. This complicates any attempt to create a phylogeny which includes all viruses, because not all viruses share a common ancestor. However, it is relatively easy to determine evolutionary relationships between viruses with very similar genomes. For example a detailed phylogeny of the SARS-CoV-2 is continuously being updated as the virus mutates and evolves.

The World Health Organization (WHO) became aware of a new strain of the SARS-CoV-2 coronavirus in January of 2020. After this first detection, the strain’s prevalence continued to expand. On March 11, 2020, the WHO declared COVID-19 to be a global health pandemic, and by the end of 2021, COVID-19 was responsible for the deaths of approximately 5 million individuals, worldwide. 

Throughout 2020, researchers worked on developing a COVID019 vaccine, and across 2021, it became available in many countries. As of early 2023, roughly 70% of the world’s population had received one dose of the COVID-19 vaccine. In subsequent months, as new viral strains emerged, the vaccine was updated in an attempt to optimize its efficiency.