In addition to being more transmissible, the variant now dominant in the UK may be about 30 percent more deadly than previous strains, but much uncertainty remains.
The so-called 614G mutation in the viral spike protein does not appear to cause more severe cases of COVID-19, but multiple studies indicate that it could be more contagious.
A handful of factors tip the scales in making a virus more likely to trigger a disruptive global outbreak. Right now, scientists tend to rank influenza, coronaviruses, and Nipah virus as the biggest threats.
Analyses of more than 300 ancient human genomes show that Hepatitis B virus has infected humans for at least 4,500 years and has much older origins than modern viral genomes would suggest.