Everyone’s been there. You’re on a packed bus at rush hour and the person next to you begins a valiant attempt to cough up their lung. You attempt to distance yourself but are blocked in every direction by a wall of jacket-clad businessmen and pram-pushing families, who are also looking worriedly at the the hacking mess of tissues in the window seat.
Eventually you have to give in and hope that the cold or (if you’re really unlucky) flu you have probably caught won’t be too bad. But what if the aerosolised particles that this unfortunate person coughed into the enclosed atmosphere of the bus contained a far more sinister bug than those that cause the common cold?
Airborne diseases are more panic-inducing than those that are transmitted by, say, direct contact with bodily fluids or faecal matter. This is because breathing is something we all have to do and, like the bus scenario but many times worse, there is a particular brand of horror associated with being trapped in a confined space in which the very air you inhale could be lethal.
But can a virus or bacteria just adopt this airborne lifestyle at the drop of a hat? In short, no. Although little is known about their exact nature, it is clear that a host of changes need to happen in order for a pathogen to ‘go airborne’. Not only would the aspiring infective agent need to replicate in sufficient quantities in the lungs, it would also need to be able to survive as an aerosol in the environment and then have the ability to infect the lungs of the unfortunate recipient.
This is why the inundation of reports over the last year claiming that the Ebola virus is close to accomplishing this horrifying feat should be taken with a hefty pinch of salt (see this great blog post for non-sensationalist information on Ebola transmission).
Another aspect needs to be taken into account: from the virus’ perspective, is it ‘worth’ going airborne? If the mutations required to allow this mode of transmission detract from the virus in some other way then these mutations will not catch on. This is just natural selection.
A relevant example of this balancing act has been shown in flu. Pandemic strains of flu like H5N1 and H7N9 (see this previous Pandemic Potential post for more info. on these viruses) are very effective at transmitting between birds through the air, but when they try to infect a human they find that their tools are somewhat less effective. Ferrets are considered to be good models for human influenza, and they were used for a fascinating and controversial experiment into the transmission of H5N1. In this experiment, the virus was given the ability to transmit through aerosols among the ferrets. It was found (unsurprisingly) that a range of different mutations was required to allow this to happen. Interestingly though, the resulting aerosol-ready virus had lost its ability to kill the ferrets. The gain of a new method of transmission had resulted in a loss of the virus’ virulence.
“We have been studying viruses for over 100 years, and we’ve never seen a human virus change the way it is transmitted”
To summarise then, I’ll refer to a very convincing article by the eminent virologist Vincent Racaniello. In case you had any doubts about the difficulties associated with becoming airborne, Raciniello explains how no virus has ever been able to change the way it is transmitted. HIV has infected millions through sex and contaminated needles without once turning round and saying “Hey, I feel like transmitting through the air now!” This does not just apply to the airborne route but any change in the method of transmission. The key point is that these changes are so fundamental, and would require such gargantuan shifts in the very nature of the virus, that the chances of them happening are remote.
It is true that viruses are mutating all the time and that undoubtedly there is a certain combination of mutations that would allow a virus like Ebola to ‘go airborne’, however I hope you can see now that the likelihood of this actually happening is vanishingly small.