In January, Chinese virologists isolated the virus that causes Covid-19. Earlier this month, a team of virologists gave this new virus a new name: SARS-CoV-2.

To do so, they had to move the virus to the head of a very, very long line.

In recent years, scientists have discovered that the world of virus diversity — what they sometimes call the virosphere — is unimaginably vast. They have uncovered hundreds of thousands of new species that have yet to be named. And they suspect that there are millions, perhaps even trillions, of species waiting to be found.

“Suffice to say that we have only sampled a minuscule fraction of the virosphere,” said Edward Holmes of the University of Sydney in Australia.

With the discovery of viruses in the late 1800s, scientists soon recognized that different species caused different diseases — rabies and influenza, for example. Later, virologists learned how to recognize new kinds of viruses by growing them in labs, where subtler biological features emerged.