similar number of people. If an infection spreads from person-to-
person, creating a chain of cases, we refer to it as ‘propagated
transmission’. However, propagated outbreaks don’t necessarily
follow the clockwork pattern of the reproduction number, growing by
the exact same amount each generation. In 1997, a group of
epidemiologists proposed the ‘20/80 rule’ to describe disease
transmission. For diseases like and malaria they’d found that 20
per cent of cases were responsible for around 80 per cent of
transmission.[51] But like most biological rules, there were some
exceptions to the 20/80 rule of transmission. The researchers had
focused on sexually transmitted infections (STIs) and mosquito-borne
infections. Other outbreaks didn’t always follow this pattern. After the
2003 epidemic – which had involved several instances of mass
infection – there was renewed interest in the notion of
superspreading. For , it seemed to be particularly important: 20
per cent of cases caused almost 90 per cent of transmission. In
contrast, diseases like plague have fewer superspreading events,
with the top 20 per cent of cases responsible for only 50 per cent of
transmission.[52]
In other situations, an outbreak may not be propagated at all. It
may be the result of ‘common source transmission’, with all cases
coming from the same place. One example is food poisoning:
outbreaks can often be traced to a specific meal or person. The most
infamous case is that of Mary Mallon – often referred to as ‘Typhoid
Mary’ – who carried a typhoid infection without symptoms. In the early
twentieth century, Mallon was employed as a cook for several families
around New York City, leading to multiple outbreaks of the disease
and several deaths.[53]
During a common source outbreak, cases often appear within a
short period of time. In May 1916, there was a typhoid outbreak in
California a few days after a school picnic. Like Mallon, the cook
who’d made the ice cream for the picnic had been carrying the
infection without knowing.