Researchers overlooked airborne diseases for centuries — then COVID-19 changed everything

A fascinating exploration of microbes that can travel through the air reveals how the pandemic marked a turning point for a crucial research field

Air-Borne: The Hidden History of the Life We Breathe Carl Zimmer Dutton (2025)

In the first months of the COVID-19 pandemic in 2020, one of the most pressing questions for researchers was how the SARS-CoV-2 virus was transmitted. Some physicians and public-health specialists argued that the virus spread solely in wet droplets from coughing. Others — backed by aerosol scientists — thought that it also spread through the air.

In Air-borne, acclaimed science writer Carl Zimmer examines why it took more than a year of vociferous debate to establish that the second theory was correct. His book is an enthralling historical exploration of the role of airborne microorganisms in disease, and examines why these pathogens have been overlooked for centuries.

The beginning of ‘aerobiology’, as -Zimmer labels it, had its roots in ancient Greece. -Hippocrates, a physician and philosopher, was the first to propose that a miasma — “an invisible corruption of the air” — caused disease. The concept endured: from the fourteenth century onwards, miasmas were often wrongly blamed for plague outbreaks that devastated cites across the world. Now, we know that the disease was caused by a bacterium carried by fleas that live on rodents.

A long history

In the sixteenth century, Italian physician -Girolamo Fracastoro wrote a book about tiny, invisible ‘seeds’ that could multiply in a host and spread through the air. Proponents of his theory, known as contagionists, would lock horns with miasmatists — who maintained that the air itself was the cause of disease — for the next 300 years. It wasn’t until the late 1800s — after physician Robert Koch proved that anthrax was caused by bacteria — that the idea that microbes cause infectious diseases became accepted and miasma theory was rejected.

In the 1930s, scientists began seriously considering the idea that microbes can spread disease through the air. Mildred Wells was an insightful, rigorous researcher who found that poliovirus was airborne. Her husband William — inspired by his wife’s work — pivoted from studying waterborne to airborne pathogens, and made the revolutionary discovery that bacteria released in sneezes could linger in the air of a classroom after the students had left.

In their landmark theory of airborne disease transmission, they proposed that diseases not only are spread through the droplets that are dispelled by coughs and sneezes and fall quickly to the ground, but also can be transmitted in microdroplets, now known as aerosol particles. These tiny particles, they argued, are released with every exhalation, settle very slowly and drift long distances.

Mildred went on to show that disinfecting air in schools with ultraviolet light reduced cold and flu incidence. William, meanwhile, found that hospitalized people with tuberculosis could infect guinea pigs through bacteria in aerosol particles — much to the annoyance of hospital staff, who for years had to run extra tuberculosis tests and autopsy dozens of guinea pigs. Unfortunately, the duo’s work was largely dismissed — perhaps owing to concern about reviving the discredited miasma theory and their “cantankerous and wearying” personalities.

Reading Air-borne, I felt frustrated by how long it took for researchers to accept that pathogens could spread through the air. Zimmer synthesizes research from various fields in a way that makes airborne transmission seem obvious, illustrating how scientific siloing inhibits progress. For example, plant pathologists knew from the early 1900s that the fungus that causes rust on wheat could spread to farms hundreds of kilometres away. Yet physicians — focused on the idea that -diseases spread through contact with infected people or contaminated objects — vehemently rejected the idea of airborne transmission indoors, even as late as the 1950s.