Modern day Typhoid Marys are on the loose! Carrie Arnold describes the dangers for Slate:
If germs hung a recruiting sign for their hosts, it would probably be a version of the World War I poster of Uncle Sam pointing: We want YOU to help us reproduce. All hosts were equally eligible for service, infectious-disease researchers thought. Assuming the recruits weren’t immune due to a prior infection or vaccination, anyone should have roughly the same potential to spread a disease’s pathogens. But then came severe acute respiratory syndrome, or SARS.
This pandemic started as just another strange pneumonia from southern China, but in 2003 it turned into a global outbreak that infected 8,098 people and killed 774. Key to the disease’s spread, researchers found, was a small but crucial portion of the population that became known as “superspreaders,” people who transmitted the infection to a much greater than expected number of new hosts. The more scientists learn about superspreaders, the more they are beginning to realize that this tiny segment of the population is the driving force behind the emergence and spread of infectious diseases.
Perhaps the most infamous superspreader in history was Mary Mallon, aka Typhoid Mary. An Irish cook in New York City in the 1900s, she was chronically infected with Salmonella typhi. Although the infection didn’t cause any symptoms for Mallon, she did excrete large numbers of typhoid bacteria in her feces. Her career as a cook made it easy to transmit fecal bacteria to customers through the food she prepared. She infected about 50 people and killed several (official counts vary) before she was arrested and jailed for refusing to give up her career as a cook.
To epidemiologists, people like Typhoid Mary were seen as an anomaly and not the main drivers of infectious disease spread. Instead, epidemiologists focused on a number they called R0, the average number of people a single person could infect. The value of R0 depends on three main factors: the number of susceptible individuals in the population, the number currently infected, and those resistant to infection. If the value of R0 is less than one, it means that each individual infects less than one other person, and the outbreak will ultimately die out. However, if the value of R0 is greater than one, the disease has the potential to spread.
When you look across a large population, R0 is a good estimate of whether and how a particular infection might spread. At an individual level, it turns out, R0 is less accurate. Most people won’t spread the disease at all, but a very few people will spread the infection to tens or even hundreds of others.
As computer models of infectious disease grew more powerful and precise, scientists began to realize that a lot of infectious disease spread is due to superspreaders…
[continues at Slate]