I turn around in time to see the doors close behind Grace, and that’s it. It’s Nia and me now. I scour the kitchen for more paper and a pen, nibble on a pear, and start my new job counting cooked meals, leftover turkey, and endless jars of peanut butter.

It’s good to know what we have. It’s good to have a purpose. If I can’t be with my sister, I’ll focus on doing the best job I can—and I think it would make Leah proud.

WHAT IS YERSINIA PESTIS? AN INTRODUCTION

• Y. pestis is a gram-negative bacterium, widely known by the disease it causes: the plague. The plague is known to take three forms: bubonic, septicemic, and pneumonic.

• Pneumonic plague arises from infection in the lungs. It can develop from inhaling infectious droplets or from other untreated forms of plague that spread to the lungs.

• Y. pestis has been on the CDC’s National Notifiable Diseases Surveillance System since the system’s inception. With outbreaks in at least a dozen cities, Y. pestis is a nightmare scenario come true for scientists.

• Some scientists believe the current and multidrug-resistant P21 variant is not naturally occurring but has been genetically weaponized, but as of yet, no evidence exists to back up that claim.

Historically speaking, the plague may be best known as having caused the 14th century Black Death, a pandemic that killed an estimated 30 to 60 percent of the population of Europe, North Africa, and Asia. But there have been multiple plague pandemics throughout history, as well as many localized outbreaks.

In modern times too, plague cases continue to occur, and the plague is said to be endemic in at least two dozen countries across the world. It has always been considered a threat to human health, but up until recently, it was a controlled threat and one with an effective treatment.

Following an outbreak of the plague in several cities across the country, doctors, hospitals, and scientists are discovering that this current strain of Y. pestis—Yersinia pestis modernis, with its current variant P21, which presents predominantly as pneumonic plague—has developed a multidrug resistance. It cannot be treated with the antibiotics normally used to treat victims.

Multidrug resistance in Y. pestis is not new. It was first discovered last century, and it’s been a source for concern for scientists. But while there have been multiple cases since, they have never occurred on this scale.

Additionally, the way infections are spreading suggests that this variant may be more virulent than what we were used to. While it was previously understood that transmission of infected droplets is the only way that plague can spread between people, and that people therefore had to have been in close contact with each other, initial case spread also suggests infections following fleeting contacts.

Without early treatment, the pneumonic plague has a near 100 percent fatality rate. Even with early treatment, the IFR remains high. But compared to fatality rates of other recent outbreaks, it appears that this plague variant—at least—is not as immediately deadly as the original strain.