The emergence of SARS-CoV-2, the virus behind COVID-19, introduced a range of new terms to the public, from “fomite” to “social distancing.” Refer to our guide from 2020 for more details. Now, a new report from the World Health Organization proposes new terms and definitions, along with a revised perspective on airborne pathogen transmission. Jeremy Farrar, WHO’s chief scientist, views this document as a foundational tool. By creating a shared vocabulary and approach, he and his team aim to align public health professionals, minimize confusion, and improve future infection control.



#1. People with respiratory illnesses often exhale infectious particles. What is the size range of these “infectious respiratory particles”?

#2. Infectious respiratory particles move through the air in what WHO calls a “turbulent puff cloud” — a mix of gases and respiratory particles from the lungs. What happens when this cloud slows down sufficiently?

#3. The new WHO report identifies two ways infectious respiratory particles move through the air. The first is airborne transmission or inhalation. Where might the particles enter someone’s body in this scenario?

#4. Another way that infectious respiratory particles can be transmitted through the air is “direct deposition.” What defines this mode of transmission?

#5. Many factors influence how particles spread and their potential impact, including evaporation, environmental conditions, and speed of expulsion. Which of the following activities leads to a “more forceful expiration” in WHO’s terms?

#6. If you’re exposed to an airborne pathogen, are you automatically infected?

#7. Transmission of infectious respiratory particles is more likely indoors than outdoors. Why?

#8. The new report states it’s not advisable to categorize particles into aerosols (smaller) and droplets (larger). Why is this?

#9. The report also discusses “direct contact” and “indirect contact” as non-airborne transmission modes. How do these modes differ?