The Rules of Contagion

Illustration of an outbreak curve that grows exponentially until

everyone is affected

Back in real life, there are a few infections that affect their hosts in
a way that increases transmission. Animals infected with rabies are
often more aggressive, which helps the virus to spread through bites,
[55] and people who have malaria can give off an odour that makes
them more attractive to mosquitoes.[56] But such effects generally
aren’t large enough to overcome declining numbers of susceptibles in
the later stages of an epidemic. What’s more, many infections have
the opposite effect on behaviour, causing lethargy or inactivity, which
reduces the potential for transmission.[57] From innovations to
infections, epidemics almost inevitably slow down as susceptibles
become harder to find.

R to study a whole range of outbreaks, but
as his models became more complicated, the mathematics became
trickier. He could outline the transmission processes, but he couldn’t
analyse the resulting dynamics. That’s when he turned to Hilda
Hudson, a lecturer at London’s West Ham Technical Institute.[58] The
daughter of a mathematician, Hudson had published her first piece of
research in the journal Nature when she was ten years old.[59] She
later studied at the University of Cambridge, where she was the only
woman in her year to get first class marks in mathematics. Although
she matched the results of the male student who ranked seventh, her
performance wasn’t included in the official listing (it wasn’t until 1948
that women were allowed to receive Cambridge degrees[60]).
Hudson’s expertise made it possible to expand the Theory of
Happenings, visualising the patterns the different models could
produce. Some happenings simmered away over time, gradually
affecting everyone. Others rose sharply then fell. Some caused large
outbreaks then settled down to a lower endemic level. There were
outbreaks that came in steady waves, rising and falling with the
seasons, and outbreaks that recurred sporadically. Ross and Hudson
argued that the methods would cover most real-life situations. ‘The