The book begins with the original pioneers of germ theory, focusing on Louis Pasteur and Robert Koch in establishing that microorganisms cause disease. Zimmer walks the reader through some of their most famous experiments, which became a foundation for proving microbial causation of disease. Along the way, Zimmer introduces early debates on germ theory, including Pettigrew’s opposition, where he argued that diseases were caused by gaseous emissions from microbes rather than the microbes themselves. Though ultimately disproven, Pettigrew made lasting contributions, pioneering carbon dioxide measurement and establishing the Pettigrew limit (1,000 ppm of the gas), which is still used today as an indoor air standard. Zimmer also discusses the early development of masks, first for epidemic control in China (~1910) and later during the 1918 flu pandemic, even though their effectiveness was not well understood at the time. While Zimmer provides a great history of these key figures and their contributions, he also takes the reader through the evolving science of airborne disease transmission.
The 1930s and 1940s were a key period for airborne transmission research, though much of it was later forgotten and rediscovered. Among the early pioneers were William and Mildred Wells, who conducted studies at the Germantown School and Loch Raven Hospital, including early work on UV light disinfection. Richard Riley expanded on their research, developing mathematical models for airborne disease spread. Harold Egerton at MIT, who produced early sneeze photographs, provided further evidence of airborne transmission. Another key figure in airborne research was Peter Tsai, whose ideas on electrostatic filtration led to the development of the N95 respirator by 3M. Zimmer highlights how these early discoveries were largely ignored for decades, delaying progress in recognizing airborne transmission.
Despite these advancements, airborne transmission remained controversial. Charles V. Chapin, a major public health authority, argued that most diseases spread via large droplets rather than aerosols. This dogma became embedded in medical training and public health policy, despite evidence suggesting otherwise. The book details early debates on this topic that shaped how airborne transmission was understood and, in some cases, dismissed. Zimmer details early debates, including experiments in Toronto, where one scientist studied exhalation and sneezing at different heights. The findings were interpreted as supporting the large droplet model, reinforcing skepticism toward airborne spread.
Zimmer also examines the intersection of public health and biodefense. The book describes the history of biowarfare research in the U.S. and the debate between Alexander Langmuir and Theodore Rosebury, which mirrored, to some degree, the nuclear weapons divide between Oppenheimer and Teller. The book discusses how the CDC was originally formed as a quasi-military entity, with a dual role in civilian health monitoring and biological weapons surveillance. Zimmer contrasts the US program with that of the Soviet Union, which experienced multiple accidents, including the Sverdlovsk anthrax release. Zimmer critiques how biodefense policies under Bush and Clinton focused more on bioweapons threats (e.g., Iraq) than pandemic preparedness. Bush’s 2005 Biodefense Strategy, following the 2001 anthrax attacks, aimed to prepare for biological threats, including stockpiling N95 respirators and building a manufacturing facility—a plan that never materialized. The stockpile was depleted during H1N1 (2009) and never replenished, leaving the U.S. unprepared for COVID-19.
As the book builds toward the COVID-19 pandemic, Zimmer provides a historical overview of pandemics, including the 1918 flu, SARS, and MERS, emphasizing how each should have better informed the COVID-19 response. He details Event 201, a 2019 pandemic simulation that accurately predicted many of the failures that later occurred. Despite these warnings, when COVID-19 emerged, the WHO and CDC were slow to acknowledge airborne transmission, instead focusing on surface cleaning and six-foot distancing, which failed to fully address how the virus spread. Zimmer highlights how airborne transmission experts, including Lindsay Marr (V Tech), Bora Berry (MIT), and Donald Milton (University of Maryland) recognized early that COVID-19 was airborne, yet their findings were ignored. During this period, an informal group of 36 scientists petitioned the WHO to update its guidance, but their efforts were dismissed.
Zimmer also critiques inconsistent messaging in the pandemic, citing remarks by Donald Trump and Joe Biden that initially acknowledged the airborne spread but were later walked back. A WHO official even admitted COVID-19 was airborne before correcting himself, contributing to public confusion and delayed mitigation efforts. Eventually, the WHO quietly revised its stance without addressing its initial resistance. Zimmer concludes by discussing overlooked solutions, such as ventilation improvements, the Corsi-Rosenthal box, and far UV technology, which offer promising but underutilized mitigation strategies.
