Sept. 25: This article was updated to include information about the song “Patient Zero” after is official release.

The arrival of new Taylor Swift music has a tendency to send fans and internet sleuths on a quest for meanings hidden in her lyrics. The pop idol’s release on Friday of four new songs was no different.

In a social media post announcing the release, she highlighted one track, “Patient Zero.” Articles were dedicated this week to piecing together clues about the song.

But the term “patient zero” has its own complex history. It has become part of the cultural lexicon, used in films and novels about dangerous pathogens. Its usage became even more common during the Covid-19 pandemic, and during a recent outbreak of hantavirus.

The term is largely viewed as a misnomer in the medical and science communities, and it can invite stigma and alarm.

Here’s how it was created decades ago by accident, how its usage has changed over time, and a hint at what Swift may actually be getting at.

What is the origin story of ‘Patient Zero?’

“Patient zero” has its roots in the AIDS crisis, when health investigators were trying to track the early spread of the virus in the 1980s. A study identified one French Canadian man, Gaétan Dugas, as the link between several California cases. The researchers labeled him as “Patient O,” short for “Out of California.”