A virus that causes COVID 19 (see COVID 19 pandemic). Its airborne nature has not been properly recognized. Greenhalgh2021ten reviews evidence for Airborne disease transmission.
Immunity to SARS-CoV-2 variants of concern
Coronaviruses are very large and complex compared with other RNA viruses (around four times the size of the hepatitis A virus genome), and their replication fidelity must therefore be higher. Despite this, once a pathogen has been allowed to infect more than 100 million people, it is not a surprise that sequence variants with a selective advantage emerge.
Ultimately, the best defense against emergence of further variants of concern is a rapid, global, vaccination campaign—in concert with other public health measures to block transmission. A virus that cannot transmit and infect others has no chance to mutate.