How many airborne contagious diseases exist? No one knows the precise answer to that question, so let me give you an easier one. How many airborne contagious diseases are recognized by the CDC?1 Let me give you your options:
What do you think? The answer is actually a! Now for a follow-up question.
What are the three recognized airborne contagious diseases?
a. Legionnaires disease, pontiac fever, and hypersensitivity pneumonitis
b. Aspergillosis, valley fever, and histoplasmosis
c. Influenza, common cold, SARS
d. Chicken pox, tuberculosis, and measles
The right answer is d! That may surprise you. Answers a and b include airborne diseases that are not contagious. I’d bet most people guessed c. Although these agents will travel through the air for a short time, their transmission is not classified as being airborne strictly speaking. More on that in a minute. The correct answer is d because chicken pox, tuberculosis and measles are all contagious and have an airborne transmission route.
Are you still upset, thinking that c is a valid answer? We need to distinguish between large droplet transmission and airborne transmission. Large droplets are generated when an infected individual talks, coughs or sneezes. The large droplets are traditionally thought to be greater than 5 microns in diameter and travel less than a meter. True airborne transmission occurs when the infectious agents travel on smaller particles, less than 5 microns, for greater distances. These small particles are called droplet nuclei. They form when the saliva or sputum they were traveling on evaporates. They are therefore the dried residual of respiratory droplet.
The current mainstream thought is that influenza, common cold and SARS are transmitted through large droplets, and chicken pox, tuberculosis and measles are transmitted through droplet nuclei. I will say this… there is some compelling evidence that influenza, common cold and SARS may all be able to travel in the air beyond a meter.
If you got the two quiz questions correct, let me know in the comments section! Have a great weekend!
I. Center for Disease Control and Prevention, 2007 Guideline for Isolation Precautions-Preventing Transmission of Infectious Agents in Healthcare Settings