Why is it important to identify “Patient Zero” in disease outbreaks?

I recently watched the excellent, Contagion, a film by Steven Soderbergh, which gives a harrowing, scientifically realistic depiction of the outbreak of a highly contagious, often fatal disease. Highlighting both the potentially rapid spread of virus outbreaks and the influence of social media scaremongers/conspiracists in today’s interconnected world, the film is a great advert for why we need our politics informed by scientists’ thinking more than ever.

One thing that the film really helps dramatize is the reason why epidemiologists are so keen to identify “Patient Zero” after the outbreak of an infectious disease. As you might have guessed, Patient Zero–or the index case as it’s clinincally known–refers to the first person who shows documented symptoms of the disease. In Contagion, that unlucky person is Gwynth Paltrow, who comes back to the US after a business meeting in Hong Kong, stopping off for a spot of extra-marital play during her layover in Chicago.

So, why do the infectious disease experts trip over themselves to find Patient Zero? Why don’t they just deal with the cases at hand? Two main reasons. One, the earlier in the outbreak of a disease that the scientists can identify the first human carrier, the better their chances of controlling and dampening the outbreak. By determining exactly where Patient Zero travelled and who they came into contact with, epidemiologists are often able to track the spread of an infectious disease, and undertake procedures to isolate and treat the people who might be carriers. If this is done early enough, before the disease has spread to too many individuals or to “supercarriers”–human vectors who, by virtue of their profession or disposition are likely to infect many others–the outbreak can be artificially “shutdown” before it runs its natural devastating course. It really is a race against time.

While the first reason concerns the onward trajectory of the disease, the second looks back to its origin. By identifying Patient Zero, epidemiologists have a strong chance of locating the exact place where the first infection happened. Often harmful viruses exist in some natural “reservoir” such as a population of wild animals like bats or civet cats, and somehow this virus ends up coming into contact with Patient Zero. Learning about the environment in which Patient Zero fell sick can often lead to the identification of the source of the virus, which allows preventative steps to be taken to stop future epidemics. In Contagion the scientists eventually discover that “The wrong bat met the wrong pig”, leading to poor Gwynth’s infection when a chef shakes her hands after handling contaminated pork.

In a future post I’ll look at the basic mathematical models that epidemiologists use to predict disease outbreaks, and why vaccinating the entire population isn’t necessary!


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