southeast outwards. It took about three months to move 2,000
kilometres across the eastern US, which works out at a speed of just
under 1 km/h. On average, you could have outwalked it.[21]
Although long-distance flight connections are important for
introducing viruses to new countries, travel within the US is
dominated by local movements. The same is true of many other
countries.[22] To simulate these local movements, researchers often
use what’s known as a ‘gravity model’. The idea is that we are drawn
to places depending on how close and populous they are, much like
larger, denser planets have a stronger gravitational pull. If you live in
a village, you might visit a nearby town more often than a city further
away; if you live in a city, you’ll probably spend little time in the
surrounding towns.
This might seem like an obvious way to think about interactions
and movements, but historically people have thought otherwise. In
the mid-1840s, at the peak of Britain’s railway bubble, engineers
assumed that most traffic would come from long-distance travel
between big cities. Unfortunately, few bothered to question this
assumption. There were some studies on the continent, though. To
work out how people might actually travel, Belgian engineer Henri-
Guillaume Desart designed the first ever gravity model in 1846. His
analysis showed that there would be a lot of demand for local trips,
an idea that was ignored by rail operators on the other side of the
channel. The British railway network would probably have been far
more efficient had it not been for this oversight.[23]
It can be easy to underestimate the importance of social ties.
When Ronald Ross and Hilda Hudson wrote those papers on the
‘theory of happenings’ in the early twentieth century, they suggested it
could apply to things like accidents, divorce and chronic diseases. In
their minds, these things were independent happenings: if something
happened to one person, it didn’t affect the chances of it happening
to someone else. There was no element of contagion from one
person to another. At the start of the twenty-first century, researchers
started to question whether this was really the case. In 2007,
physician Nicholas Christakis and social scientist James Fowler
published a paper titled ‘The Spread of Obesity in a Large Social