Ebola: infectious versus contagious. (plus some info on airborne transmission)

By Dr. Yvette Lu


Recently, I talked on Breakfast Television about some of the basics of the Ebola Virus and why you are highly unlikely to get Ebola here in North America (or in any developed country).


Here’s the video:

Video link: http://youtu.be/63eoo_Fhbsc?list=PLzxx5hDVqT3u0N5QAIEGCO9MK9y9CIeK_


I didn’t have time to get into the concepts of infectious versus contagious.

There has been a lot of confusion about this on the internet and in the media. To understand Ebola and its transmission, it’s important to understand the difference between the concepts of infectious and contagious.

Ebola is very infectious. It is NOT very contagious. What?! Aren’t they the same??



How infectious a virus (or bacteria or other disease causing pathogen) is describes how many particles it needs to cause disease.

Ebola is very infectious. As few as 1-10 particles of Ebola entering through a mucous membrane (like the inside of your eye or nose) can give you the disease.

Contrast this to some strains of Salmonella and E. coli, which require thousands of organisms to establish an infection in humans.

How contagious a virus is describes how easily it spreads from person to person.

Ebola is NOT very contagious. On average, one patient with Ebola will spread it to two other people. Transmission of Ebola requires direct contact with blood and body fluids. It is NOT spread by airborne transmission.

Measles is 9 times more contagious than Ebola. One person with measles on average spreads it to 18 people if the population is not immunized. Measles is much more contagious because it can aerosolize and travel through the air on dust particles for long distances (airborne transmission). The complication rate of measles is 1/1000 for encephalitis (inflammation of the brain) and the chance of dying of measles is about 1/3000. You can see now why public health officials and doctors get so anxious when people don’t immunize their children. Measles can spread very rapidly in a vulnerable community and cause a lot of trouble.

Scientists have been studying Ebola since 1976, and there have been over 20 outbreaks in the past 40 years. They have done studies of household contacts of people with Ebola, as well as studies looking at transmission to health care providers. All the studies have shown that although Ebola is infectious, it is not very contagious.

Ebola, because it requires direct contact with blood and body fluids, is difficult to pass from person to person, but because it is infectious, it doesn’t take very much fluid to pass it on, which is how the nurses (who were working with the body fluids – urine, vomit, diarrhea – of a person with Ebola) caught it. Because Ebola is not easily spread by casual contact, household contacts of Thomas Duncan (the Ebola patient) including his girlfriend, son, and two people who lived with him, did not get Ebola. People do not normally touch others’ body fluids. Plus, there is some question about whether or not the nurses were adequately protected early on during the treatment of Mr Duncan (their necks were initially exposed or they may have used the isolation gear incorrectly).


This is a great chart here that shows the contagious concept graphically:

Source: http://www.npr.org/blogs/health/2014/10/02/352983774/no-seriously-how-contagious-is-ebola


Also, we should remember that although we need to be vigilant about and wary of Ebola, HIV/AIDS is still, by far, the leading cause of death in Africa. This is an excellent chart and analysis in this article that explains this (myth number 6):

Source: http://www.vox.com/2014/10/1/6880393/ebola-virus-outbreak-disease-myths-facts-debunked-america


Finally, for those still wondering if Ebola will mutate to become airborne, the answer is no. It’s not impossible, of course, but it is highly unlikely. In all our time studying viruses, we have never observed a virus to change its mode of transmission. It’s too fundamental to its way of life. As quoted in this New Yorker article (http://www.newyorker.com/magazine/2014/10/27/ebola-wars), it’s like asking if Zebras will fly. Not impossible, but highly unlikely. A better question to ask is if Ebola can mutate to become better at what it does, sort of like asking if Zebras can change to run faster. For Ebola, this could involve becoming more virulent (replicating faster) or finding a way to hide from our diagnostic tests.

For now, the best thing for everyone to do is to educate themselves about the science, calm the fear, and get the flu vaccine… because flu-related complications cause an average of 23,000 deaths in the US and 3500 deaths in Canada each year. Hmmm… now how many deaths has Ebola caused in North America so far?




More info about Ebola can be found here:

Dr Yvette Lu
twitter: @yvettelu