opposite the doctor, beside him, or along the corridor. As the disease
spread, there was an urgent need to understand the new virus
causing it. Scientists didn’t even know basic information like the
delay from infection to appearance of symptoms (i.e. the incubation
period). With cases appearing across Southeast Asia, statistician
Christl Donnelly and her colleagues at Imperial College London and
in Hong Kong set out to estimate this crucial information.[56]
The problem with working out an incubation period is that we
rarely see the actual moment of infection. We just see people
showing up with symptoms later on. If we want to estimate the
average incubation period, we therefore need to find people who
could only have been infected during a specific period of time. For
example, a businessman staying at the Metropole had overlapped
with the Chinese doctor for a single day. He fell ill with six days
later, so this delay must have been the incubation period for his
infection. Donnelly and her colleagues tried to gather together other
examples like this, but there weren’t that many. Of the 1,400
cases that had been reported in Hong Kong by the end of April, only
57 people had a clearly defined exposure to the virus. Put together,
these examples suggested that had an average incubation
period of about 6.4 days. The same method has since been used to
estimate the incubation period for other new infections, including
pandemic flu in 2009 and Ebola in 2014.[57]
Of course, there is another way to work out an incubation period:
deliberately give someone the infection and see what happens. One
of the most infamous examples of this approach occurred in New
York City during the 1950s and 1960s. The Willowbrook State
School, located on Staten Island, was home to over 6,000 children
with intellectual disabilities. Overcrowded and filthy, the school had
frequent outbreaks of hepatitis, which had led paediatrician Saul
Krugman to set up a project to study the infection.[58] Working with
collaborators Robert McCollum and Joan Giles, the research
involved deliberately infecting children with hepatitis to understand
how the infection developed and spread. As well as measuring the
incubation period, the team discovered they were actually dealing