Koch has failed with the mosquito in Italy,’ Manson wrote, ‘so you
have time to grab the discovery for England.’
Eventually Koch did publish a series of malaria studies, which fully
credited Ross’s work. In particular, Koch suggested that children in
malarial areas acted as reservoirs of infection, because older adults
had often developed immunity to the parasite. Malaria was the latest
in a line of new pathogens for Koch. During the 1870s and 1880s, he
had shown that bacteria were behind diseases like anthrax in cattle
and tuberculosis in humans. In the process, he’d come up with a set
of rules – or ‘postulates’ – to identify whether a particular germ is
responsible for a disease. To start with, he thought that it should
always be possible to find the germ inside someone who has the
disease. Then, if a healthy host – like a laboratory animal – was
exposed to this germ, it should develop the disease too. Finally, it
should be possible to extract a sample of the germ from the new host
once they fall ill; this germ should be the same as the one they were
originally exposed to.[40]
Koch’s postulates were useful for the emerging science of ‘germ
theory’, but he soon realised they had limitations. The biggest
problem was that some pathogens don’t always cause disease.
Sometimes people would get infected but not have noticeable
symptoms. Researchers therefore needed a more general set of
principles to work out what might be behind a disease.
For Austin Bradford Hill, the disease of interest was lung cancer.
To show that smoking was responsible, he and his collaborators
would eventually compile several types of evidence. He’d later
summarise these as a set of ‘viewpoints’, which he hoped would help
researchers decide whether one thing causes another. First on his list
was the strength of correlation between the proposed cause and
effect. For example, smokers were much more likely to get lung
cancer than non-smokers. Bradford Hill said this pattern should be
consistent, cropping up in different places across multiple studies.
Then there was timing: did the cause come before the effect?
Another indicator was whether the disease was specific to a certain
type of behaviour (although this isn’t always helpful because non-
smokers can get lung cancer too). Ideally there would also be