The Rules of Contagion

viruses that can spread among humans, the most successful tend to
cause longer infections (i.e. larger duration) and spread directly from
one person to another rather than via an intermediate source
(i.e. more opportunities).[47] Transmission probability can also make
a difference: bird flu viruses struggle to spread among people
because they can’t latch onto the cells in our airway as easily as
human viruses can.[48]
The same sort of adaptation can happen with online content. There
are many examples of online memes – such as posts and images –
evolving to increase their catchiness. When Facebook researcher
Lada Adamic and her colleagues analysed the spread of memes on
the social network, they noticed that content would often change over
time.[49] One example was a post that read: ‘No one should die
because they cannot afford health care and no one should go broke
because they get sick.’ In its original form, the meme was shared
almost half a million times. But variants soon emerged, with one in
every ten posts adding a mutation to the wording. Some of these
edits helped the meme propagate; when people included phrases like
‘post if you agree’, the meme was almost twice as likely to spread.
The meme was also highly resilient. After an initial peak in popularity,
it persisted in one form or another for at least two years.
Even so, there seems to be a limit to the potential contagiousness
of online content. The most popular trends on Facebook during
2014–2016 all had a reproduction number of around 2. This limit
seems to occur because the different components of transmission
trade off against each other. Some trends – like the ice bucket
challenge – involved only a few nominations per person, but came
with a high probability of transmission during each nomination. Other
content, such as videos and links, had far more opportunities to
spread, but in reality only a few friends who saw the post reshared it.
[50] Remarkably, there were no examples of Facebook content that
reached lots of friends and had a consistently high probability of
spreading to each person that saw it. This serves as a reminder of
just how weak online outbreaks are compared to biological infections:
even the most popular content on Facebook is ten times less
contagious than measles can be.