The reproduction number has become a crucial part of modern
outbreak research, but there’s another feature of contagion we also
need to consider. Because R looks at the average level of
transmission, it doesn’t capture some of the unusual events that can
occur during outbreaks. One such event happened in March 1972,
when a Serbian teacher arrived at Belgrade’s main hospital with an
unusual mix of symptoms. He’d been given penicillin at his local
medical centre to treat a rash, but severe haemorrhaging had
followed. Dozens of students and staff in the hospital gathered to see
what they presumed was a strange reaction to the drug. But it was no
allergy. After the man’s brother also fell ill, staff realised what the real
problem was, and what they had exposed themselves to. The man
had been infected with smallpox, and there would be 38 more cases
- all traceable to him – before the infections in Belgrade subsided.
[49]
Although smallpox wouldn’t be eradicated globally until 1980, it
was already gone from Europe, with no cases reported in Serbia
since 1930. The teacher had likely caught the disease from a local
clergyman who’d recently returned from Iraq. Several similar flare-ups
had happened in Europe during the 1960s and 1970s, most of them
travel-related. In 1961, a girl returned from Karachi, Pakistan to
Bradford, England, bringing the smallpox virus with her and
unwittingly infecting ten other people. An outbreak in Meschede,
Germany, in 1969 also started with a visitor to Karachi. This time it
was a German electrician who’d travelled there; he would pass the
infection on to seventeen others.[50] However, these events weren’t
typical: most cases who returned to Europe didn’t infect anyone.
In a susceptible population, smallpox has a reproduction number of
around 4–6. This represents the number of secondary cases we’d
expect to see, but it’s still just an average value: in reality there can
be a lot of variation between individuals and outbreaks. Although the
reproduction number provides a useful summary of overall
transmission, it doesn’t tell us how much of this transmission comes
from a handful of what epidemiologists call ‘superspreading’ events.
A common misconception about disease outbreaks is that they
grow steadily generation-by-generation, with each case infecting a