We’ve long known that environmental factors—from humidity and temperature to trace chemical vapors—can influence how pathogens, such as viruses, bacteria and fungi, behave once released into the air. These tiny droplets of respiratory fluid, or aerosols, carry viruses and bacteria and can float for minutes or even hours. But while we’ve been busy focusing on physical distancing and surface cleaning, a quieter factor may have been playing a much bigger role in airborne disease transmission all along: carbon dioxide (CO₂).