In a seemingly interminable year of unfortunate news, the United States crossed another grim milestone on November 18: More than 250,000 Americans have now died from COVID-19.

That's a quarter million dead friends, family, and neighbors amidst a pandemic that's only gotten worse in the past few weeks. The general public is, understandably, oscillating through emotions. One day, we hear news of promising coronavirus vaccine studies from companies like Pfizer and Moderna; the next, we're bombarded with the unfortunate realities of overrun hospitals, spiking COVID deaths, and newly implemented lockdowns.

It's unrealistic to think that the cascade of chaos which has engulfed us for the past year won't take its toll on our heads. But why is it that certain kinds of grief, in reaction to tragedy, strike us in such different ways? How does a feeling of constant unease and dread (like when a new virus decides to wreak havoc) differ from a shock incident such as a mass shooting or terrorist attack?

The reality is that there are many kinds of grief, but they tend to spring from the same roots. Sadness. Shock. Anger. Anxiety. Restlessness.

Way back in the innocent days of June, Dr. Katherine Shear, the founding director of Columbia University's Center for Complicated Grief, put it bluntly in an interview with Wisconsin Public Radio: "Grief doesn't occur in any kind of predictable way."

A sudden shock of grief can elicit an unpredictable response. The specific response varies widely from person to person. It might be anger. It might be clinical depression. There could even be manic episodes. That's just the nature of our emotional biology.

"Grief is such a powerful thing to the specific loss that you've had," said Shear. "The steps for recovering aren't orderly."

What's unique about pandemic-related grief, however, is that it's both immensely personal and societally consuming. Americans have been prevented from visiting their elderly and sick relatives; funerals have had to be scaled back or canceled; seemingly healthy and young people have had to be ventilated for weeks as they grapple with coronavirus.

Then comes the deluge of constant discussion about the virus and its ripple effects across our lives, as well as the sobering realization that we just don't know when it will all end.

Death lingers once we've lost a loved one. But it dissipates. "The presence of absence. That's how you live after someone dies," as Shear said during her radio interview.

She elaborated on what makes this particular moment so bizarre in an interview with Fortune.

"It’s very different from something like 9/11," she says. "You can’t wrap your mind around 250,000 people dead."

But the uncertainty is the real emotional killer, fueling the malaise. "We all continue to be at risk for death and loss, the two things we fear the most," says Shear. "9/11 was traumatic, but it was over after a while. This is just ongoing, and it’s turned our lives upside down."

This gets to one of Shear's passion projects: The malady that is "prolonged grief disorder."

It's a condition being evaluated and likely to be made official, but in simple terms, it's grief that's debilitating over a long period of time. And it's something Shear expects will get worse. In fact, she expects the percentage of people grappling with it will double from 10% to 20% as the pandemic continues, not just because of personal loss or existential guilt, but building anger in the midst of a hyper-politicized atmosphere.

"The persistent, pervasive yearning, longing, and sadness. An ongoing sense of disbelief," says Shear. "A lot of times we have these thoughts but quickly correct them like on 9/11. You might say, 'Why didn't I tell my brother or sister not to go to work?' But you won’t hang on to it because it doesn’t make sense. But it’s much harder to let go of not wearing a mask and not being able to be comforted by other people."