Until 2019 there were 4 Coronaviruses known to infect humans. They accounted for about 20% of colds. Last year the SARS-CoV-19 Coronavirus jumped from wild animals (probably from bats) to humans as a result of a new mutation in the virus surface protein (spike protein) which allowed this virus to enter human cells very efficiently. The virus surface (this is the spike protein) binds on to a receptor on the surface of lung cells, which is why major respiratory infection can result.
The last year has seen the tragedy of the Corona Virus pandemic where a virus new to humans has infected vast numbers of people and caused widespread illness and many deaths world-wide. While not identical to the Influenza pandemic of 1918-1920, there are many parallels between this pandemic and the current Corona virus pandemic. For example, the importance of using masks in a respiratory virus pandemic was graphically demonstrated in cities such as San Francisco in 1918. Unlike 1918, there has been not only a rapid demonstration that we are dealing with a virus, but the gene structure of the virus was uncovered quickly. An explosion of research on all aspects of the virus and the infection led to the rapid development of diagnostic tests, therapeutics and vaccines.