How COVID Reshaped Public Health Understanding of Disease Spread by Air

Viruses, wildfires, emissions from manufacturing...the list of natural and man-made airborne contaminants is long. Our air quality is affected by nearby environmental factors, as well as ripple effects from thousands of miles away. Understanding how these factors affect our health and planet is crucial. In his upcoming book, Air-Borne: The Hidden History of the Life We Breathe (Dutton), award-winning science journalist Carl Zimmer chronicles the beginnings of the field of aerobiology, exploring how microorganisms, particles and pollutants populate the air we breathe—for good and ill. In this exclusive excerpt, Zimmer shares the story of how COVID-19 affected a small choir group and, more generally, how airborne viruses are transmitted and their implications for public health.

The musicians walked onstage first: three carrying violins, one a cello. The fifth sat down at a grand piano. They were greeted with waves of sound, as the audience applauded. The applause grew stronger as dozens of singers streamed out from both wings, the men in tuxedos, the women wearing bright scarves draped over black dresses. Older singers settled down in the two rows of chairs behind the musicians. The younger ones stepped onto the low bleachers behind the chairs. Now all in place, they faced the audience. Out of the 90 people assembled onstage, four wore masks. There were about 170 people in the audience on the night of May 6, 2023: a gathering of friends, families and unconnected lovers of music from the northwestern corner of Washington State. They had traveled to McIntyre Hall in Mount Vernon for the spring performance of the Skagit Valley Chorale. One member of the choir taught fifth grade, and her tween fan army, done up in rhinestones and taffeta, buzzed overhead in the balcony. Some people in the audience wore pale blue surgical masks that fit loosely over their mouths. Others wore N95s that sealed tight. What would have seemed strange in 2019 seemed fairly normal four years later. As the applause died down, a short woman with a gray pageboy walked to the front of the stage. She introduced herself as Ruth Backlund, the president of the chorale's board, and welcomed the audience. "I have two thoughts about gratitude," she said. "I'm very grateful for Skagit County. If you look in the woods, everything is blooming. And I am grateful for all the scientists who made the vaccines that let us be here." The choir began with a hymn. At the cue of Yvette Burdick, the choir director, the singers dropped their diaphragms. They inhaled the concert hall air deep into their lungs, into the fine alveoli at the deepest tips of their airways. The oxygen in the air seeped into their bloodstreams, while carbon dioxide outgassed. The singers then let their lungs deflate, and the altered air made its way back up through their bronchi, into their tracheas and through their larynxes. Bands of muscle buzzed in the upwelling breeze and produced a spectrum of sound. The singers set their mouths into different shapes, to sculpt the acoustic waves as they escaped. The sound raced across the hall, the waves jostling molecules of air and bouncing off walls. The waves ended up in our auditory canals, making our eardrums vibrate and generating electrical signals that entered our brains, where they produced the perception of sound. The physics of the air joined us in a communion as the choir shared songs about gratitude: for the Earth, for liberation from slavery, for love. "Place me like a seal over your heart, like a seal on your arm," the vibrations told us.

Mount Vernon Choir Sees Early Surge in COVID-19 Cases

On May 5, 2023, the day before the spring concert, the World Health Organization made a major announcement. Speaking in Switzerland at a press conference, WHO director Tedros Adhanom Ghebreyesus declared that COVID‑19 was no longer a public health emergency of international concern. Three years and five months had passed since the coronavirus SARS‑CoV‑2 emerged in Wuhan, China. COVID‑19, a disease never seen before, became the worst public health disaster of modern times, infecting the majority of people on Earth. By the time Tedros made his announcement, it had killed about 25 million of them. Some of the first people in the world to get COVID‑19 stood before us on the McIntyre Hall stage. On March 10, 2020, 58 members of the Skagit Valley Chorale had become infected at a rehearsal. Before the month was out, three were in the hospital. Two of them died. The outbreak brought horror to the choir and also shock. They knew that some diseases can spread in droplets slathered on doorknobs or fired at close range in coughs and sneezes. But subsequent research would reveal that the Skagit Valley Chorale outbreak was likely spread on a song. An infected singer released an invisible cloud of droplets so tiny that they resisted gravity and floated like smoke. She did not cough or sneeze to unleash the viruses: they escaped with every breath. COVID‑19, in other words, was airborne.

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Scientists Begin Rethinking How Illnesses Spread Due to COVID-19

I am a journalist, and diseases are one of my beats. I became aware of the new virus in early January 2020, while it was still in China. By late January, a few scientists were predicting a pandemic. I started warning my friends to brace for a possible disaster. Like a paranoid doomsday prepper, I advised them to store extra toilet paper and canned food. When someone called me to plan a meeting in June, I told him meetings might not exist in June. I was right in some ways and very wrong in others. Like the Skagit Valley Chorale, I did not concern myself with the air. If I stayed a few feet away from strangers, I'd be safe from any viruses they coughed or sneezed. The droplets they expelled would fall to the floor like ball bearings. The most worrisome risk seemed to lurk on surfaces: the skin of my hands, which I washed many times a day; the grocery bags that I disinfected with Clorox wipes. Over the following months, I absorbed the growing consensus that COVID‑19 was in fact airborne. As I recognized that floating droplets could transmit the virus from one person to another, I traded Clorox wipes for a carbon dioxide monitor. Masks became a staple. I also began to think about the air differently, as a gaseous ocean in which we all live, which infiltrates our bodies, which our own bodies transform and then return to the great transparent sea, that contains exhaled viruses that can then be inhaled. But I was also left with a question: how could such a fundamental mystery about the worst public health disaster in a century go unsolved for so long? Once the pandemic passed its peak—after most people on Earth got infected, vaccinated or both—I started looking for an answer. It became clear that for thousands of years the atmosphere had been an intimate, enveloping mystery. For hundreds of generations, scholars and physicians had claimed the air itself could turn dangerous. They gave bad air an assortment of names, such as miasma. Miasmas could be caused by the stars or swamps; they could spread down a street or float for hundreds of miles. When modern Western medicine took shape in the late 1800s, scientists and doctors alike tossed miasmas aside, treating them like an embarrassing relic of the Dark Ages, a concept with as much value to medicine as bleeding patients. They knew that germs spread diseases, and they knew that germs spread primarily through food, water, sex and touch, as well as through coughs and sneezes. Germs were not airborne. But in the 1930s, a few scientists challenged this consensus. They argued that diseases could indeed spread on currents, that germs could float for hours like smoke. They recognized that airborne pathogens posed a fundamentally different threat than the one posed by short‑range coughs and sneezes. They argued that some of the worst diseases known to humanity, such as tuberculosis and influenza, spread this way. Those scientists helped create a new field: the science of airborne life. They called it aerobiology.

The aerobiologists were a motley crew. As some tracked pathogens floating inside schools and subways, others caught microbes soaring through the sky. They dazzled the world by finding spores as high as the stratosphere. The founders of aerobiology hoped their new science would unify all life of the air, whether indoors or outdoors, and make clear that the airborne diseases that afflict us are just a few species among a vast floating menagerie. When aerobiology emerged as a field of study, it generated great excitement, but within a few years it faltered. In World War II, the United States and other countries recruited aerobiologists to make biological weapons. And when the war ended, the aerobiologists kept on growing pathogens to wipe out cities and starve nations. A shroud of secrecy fell across much of aerobiology. Even today, the science is not entirely free of it. In those postwar years, some aerobiologists tried to persuade public health officials to take the threat of airborne infection seriously. They largely failed. Infectious disease experts who led the fight against out‑ breaks and prepared for the emergence of new diseases mostly ignored the aerobiologists, even when it meant accepting some basic mistakes about the physics of air. The COVID‑19 pandemic finally rattled that consensus. In so doing, it provided an opportunity to rethink our history with the air. The COVID-19 pandemic was not a fluke. It belongs to a deep history of airborne life, one that has adapted with astonishing efficiency to our species' rapid rise—from the dawn of agriculture 10,000 years ago to the rise of cities, to the Industrial Revolution, and now to the 21st century's megacities and decimated wilderness. SARS‑CoV‑2 is only one species in an airborne habitat that we largely ignore, but would do well to understand.

Fresh Air

The Skagit Valley Chorale singers finished their concert and then took their bows. As the audience cheered, I checked my CO2 monitor. The level of carbon dioxide—527 parts per million when they'd begun singing—had reached 903 parts per million. Outside, the level was hovering around 420. In our communion, we had altered the air. After the applause ended, my wife and I filed out into the high-ceilinged lobby. We congratulated Burdick and Backlund, passed by the fifth graders swarming their teacher as if she were Taylor Swift, pushed open the outer doors and walked into the night air. It was the same air we had just breathed inside McIntyre Hall, the same seamless blanket of gases. The only difference now was there was no ceiling to hold it down. I could exhale all the carbon dioxide I wanted, but my CO2 monitor would not budge. Adapted from AIR-BORNE by Carl Zimmer, to be published on 2/25/2025 by Dutton, an imprint of Penguin Publishing Group, a division of Penguin Random House, LLC. Copyright © 2025 by Carl Zimmer.