As with any virus, the Covid-19 virus will mutate over time and this requires scientists to give each variant labels to distinguish among them. Since they want to keep track of even the smallest changes, they require a system that can identify the nature of the changes and their location on the virus. But that technical name is hard for the general public to keep track of and so the WHO has adopted the Greek alphabet sequentially to label as they appear just those variants that they think most likely to affect the public and that we need to keep track of, that they call ‘variants of concern’. There are now seven of them. So the variant with the scientific name B.1.617.2 is called Delta and the latest variant B.1.1.529 is called Omicron. The earlier Alpha has the scientific label B.1.1.7 and Beta has the label B.1.352.

On 26 November, the World Health Organization (WHO) designated the strain, known as B.1.1.529, as a variant of concern and named it Omicron, on the advice of scientists who are part of the WHO’s Technical Advisory Group on SARS-CoV-2 Virus Evolution. Omicron joins Delta, Alpha, Beta and Gamma on the current WHO list of variants of concern.

Just as invading other countries is the way that Americans learn geography, the pandemic is giving them a crash course in the Greek alphabet. The order of the 24 letters in that alphabet in both upper and lower case pairs is given by Α α, Β β, Γ γ, Δ δ, Ε ε, Ζ ζ, Η η, Θ θ, Ι ι, Κ κ, Λ λ, Μ μ, Ν ν, Ξ ξ, Ο ο, Π π, Ρ ρ, Σ σ/ς, Τ τ, Υ υ, Φ φ, Χ χ, Ψ ψ, and Ω ω. How we sound the letters now is based on modern Greek usage which is different from ancient Greece. It also differs in different parts of the world. The only usage I am familiar with is in the fields of mathematics and physics where Greek letters are commonly used as symbols. Here is a guide to their common pronunciations in that limited world.

In that world, Ξ ξ is written in English as ‘Xi’ and pronounced as ‘ksaai’ or ‘zaai’. Of all the Greek letters, this is the one that I found most awkward to pronounce and the lower case one the most awkward to write and I would avoid it as much as possible.

Only the Alpha, Delta, and now Omicron variants are presently in the public eye but presumably the others that appear in the alphabet before Omicron are also still around, except that the Ν ν and Ξ ξ labels have been omitted by jumping directly from Μ μ to Ο ο .The reason is because WHO avoids naming things with labels that can cause confusion or have cultural specificity.

“‘Nu’ is too easily confounded with ‘new,’” Tarik Jasarevic, a W.H.O. spokesman, said on Saturday. “And ‘Xi’ was not used because it is a common last name.”

He added that the agency’s best practices for naming diseases suggest avoiding “causing offense to any cultural, social, national, regional, professional or ethnic groups.”

Using the Greek naming system has simplified public communication immensely.

Dr. Angela Rasmussen, a virologist at the University of Saskatchewan, said she conducted many interviews with reporters this year, before the Greek naming system was announced, and she stumbled through confusing explanations about the B.1.1.7 and B.1.351 variants. They are now known as Alpha, which emerged in the United Kingdom, and Beta, which emerged in South Africa.

“It makes it really cumbersome to talk about when you’re constantly using an alphabet soup of variant designations,” she said, adding, “Ultimately people end up calling it the U.K. variant or the South African variant.”

That’s the other big reason that the W.H.O. moved to the Greek naming system, Dr. Rasmussen said: The older naming convention was unfair to the people where the virus emerged. The agency called the practice of describing variants by the places they were detected “stigmatizing and discriminatory.”

The practice of naming viruses for regions has also historically been misleading, Dr. Rasmussen said. Ebola, for example, is named for a river that’s actually far from where the virus emerged.

“From the very beginning of the pandemic, I remember people saying: ‘We called it the Spanish flu. Why don’t we call it the Wuhan coronavirus?’” Dr. Rasmussen said. “The Spanish flu did not come from Spain. We don’t know where it emerged from, but there’s a very good possibility it emerged from the U.S.”

The W.H.O. encouraged national authorities and media outlets to adopt the new labels. They do not replace the technical names, which convey important information to scientists and will continue to be used in research.

Of course, the fact that one of the omitted letters is Ξ ξ that when written in English as ‘Xi’ also happens to be part of the Chinese leader Xi Jinping’s name has been seized upon by the usual nutcases like Donald Trump and Ted Cruz to suggest that this is evidence of the WHO’s subservience to China and their desire to avoid giving offense to him, even though the pronunciation of the letter is nothing like the Chinese name.