What are the chances you can catch a disease off an alien?

What would it take for that old SciFi classic of ‘alien disease infects human’ to come true? I wondered this while watching Babylon Five (Series 2, cannot remember the episode title), when the SomeThingOrOther aliens were being struck down by a plague, and it jumped species to the WhatEverYouCallThem aliens, and everyone was wondering if it would jump some more to humans. And their was a great wailing and gnashing of teeth until Dr McCoy (no, it is the other one in Babylon Five, isn’t it?) found the magic wibble machine that made everyone better, except for the aliens, who all died.

But in reality, how likely is this to happen? And then it occurred to me – we have the data to answer this! Not a lot of data, but some.

A few diseases have jumped from other animals to humans. I am not talking about things like TB that naturally affect several species. I am talking about H1N1 flu, that appeared in chickens and then jumped to man. Lassa Fever, Swine flu. And of course AIDS. The case of AIDS is interesting. It jumped from apes (possibly monkeys) to humans at least twice, from a population of a few hundred thousand monkeys to (let us say) a population of 100 million rural Africans. Bird flu, by contrast, apparently only jumped once between a population of billions of chickens living alongside a billion people. Swine flu is probably intermediate – less pigs, but similar numbers of humans, SARS jumped from Palm Civets to humans. WTF is a Palm Civet? Something that there are quite a lot of in Asia, apparently.

Now, there is a pattern here. We catch of diseases from each other with absurd ease. Just ask any schoolteacher. Say you catch a disease from another human once a month, out of (say) 20,000 people who thoughtlessly cough all over you in public. We catch diseases from great apes at (let’s say) one event decade per 50,000 apes x 100 million people. But it takes 1 billion chickens to cough over 1 billion humans for decades to spread one disease. Palm Civets are inbetween. Hah – the rate of disease jumping and how long ago our species went their separate, evolutionary ways are related! Other humans – 10,000 years ago, apes 10 million years, chickens 300 million years and so on. (This is why we do not catch plant viruses that often …) Playing with Excel (and blame Sara Seager for my newly acquired tendency to write equations), we find that, roughly,


where P = probability of humans catching something nasty, in a given year, A = number of animals, H = number of humans, D = time since the animal and human diverged in My, and 10 is 10 and not 2, because, no, I am not writing my numbers in binary.

So we can calculate as follows: if an equal number of humans and aliens all coughed over each other in a crowded space station, how many of them would there have to be to have an odds-on chance of one of them catching a disease from another within the timeframe of a 5-year run of a TV series, given that their last common ancestor was the creation of atoms sometime around 13.8 billion years ago? Turns out that would require 800 billion aliens and 800 billion humans. If each had an average weight of 80 kg, they would make a ball about 8½ kilometers across. Just imagine the smell in the middle …. .  Definitely would not fit into Babylon Five, or the Starship Enterprise.

Overall, then, not that likely: catching alien diseases is one terrible threat that we do not have to worry about that much. On that uncharacteristically optimistic note of good news, I say farewell for another BB.