By now, you’ve probably seen some alarming headlines about bird flu, and you may be wondering how worried you should be. I understand the uncertainty.

On the one hand, we have all spent decades hearing alarming stories about strange viruses — like MERS, Ebola, dengue and Zika — most of which don’t end up having a big effect on the U.S. On the other hand, one of those recent viruses turned into the life-altering Covid pandemic.

In today’s newsletter, I want to help you make sense of bird flu, using four questions.

Making sense of H5N1

1. What is bird flu?

It’s an influenza virus officially known as H5N1 (and sometimes called avian flu). It has been circulating for decades, and it attracted global attention in the late 1990s after an outbreak among chickens in southern China.

That outbreak was especially worrisome because it included the first documented human cases of the virus. At least 18 people were infected, six of whom died.

2. Why the recent concerns?

The virus has recently expanded in two ways: across regions and across species. Rather than being concentrated in Asia, bird flu has moved across much of the planet. And it has infected a wider variety of animals, including mammals. (This Times story explains.) Dairy cows in many parts of the United States have tested positive.