Why we still don't totally understand how diseases spread


When I was little, I read a Reader's Digest book of great disasters, which included a segment on the Black Death. One of the things the book tried to do was explain, on a child's level, why it wasn't easy to figure out that rats and fleas were the source of the plague. You couldn't just look for patterns, because there seemed to be no pattern. Half a household might drop dead while the other half only got a little sick, or remained entirely healthy. Plague doctors who handled the sick every day lived another 20 years. The real spread of disease wasn't like the movies, where one person coughing means everyone in close proximity is doomed.

One reason for the emergence of strange non-patterns like this is something called "super spreaders"—basically, some people spread disease more effectively than others. The infamous Typhoid Mary is the poster child for super spreaders, but the effect has been well-documented in a range of infectious diseases and it goes beyond the simple story of one woman who infected thousands. In fact, what makes the super spreader phenomenon so fascinating is that it isn't an anomaly at all. Super spreaders are the primary way some diseases spread. The Contagions blog—which is all about the history of infectious disease—has a great post up about this.

Eventually new models arose like the “20/80″ rule that says that 20% of cases are responsible for 80% of the transmission and formed a core ‘high risk’ group. This model works well for some diseases but not all.

For pathogens that do rely on super-spreaders, the majority of cases will not transmit the infection to anyone. This can lead to a sense of false security because it seems poorly communicated. As Galvani and May assert, “heterogeneously infectious emerging disease will be less likely to generate an epidemic, but if sustained, the resulting epidemic is more likely to be explosive”. Super-spreaders tend to beget more super-spreaders, although most of the cases they generate will still not transmit the infection to anyone. For example, a super-spreader begets 30 cases, 3 (10%) of which become new super spreaders. The rest may transmit to 0-1 people.

Super-spreading has been documented for HIV, SARS (Sudden Acute Respiratory Syndrome), measles, malaria, smallpox and monkeypox, pneumonic plague, tuberculosis, Staphylococcus aureus, typhoid fever, and a variety bacterial sexually transmitted diseases.

And that brings us back to medical mysteries because, the Contagion blog explains, we don't know exactly why some people are super spreaders and others aren't—or why some people are more vulnerable to infection than others. So far, what we have to go on is a list of well-established correlations.

Read about what makes a super-spreader at the Contagions blog.

Image: Thomas Bartholini's illustration of beak doctor from 1661. Via Wikipedia.


  1. Just curious…are most so-called “super-spreaders” usually themselves immune to a given virus, or will they inevitably deterioriate into zombieness or whatever such plague is around as quickly as others? I ask because I’ve worked in many a lab situation with sketchy biotics and ferment a lot of food at home. It seems that  a lot of people get sick around me at work and whatnot, but I never do. Any statistics on that?

  2. Here is a part of the solution. I have friends who think that its great to keep going to parties when they are sick. They hug 30 friends at a party, share drinks, who knows what else when they have a cold. They just feel that a cold is an inconvenience and they should still go out. Sometimes they say stuff like “I really should be at home since I am sick but I just could not miss soandsos birthday!” Please kiss her and give her a bigger birthday present.

    I personally rarely go out when I am sick as I am very sensitive to infecting others. 

    These friends that I have that do this rarely even understand that they are superspreaders. They are just living life. Oblivious. I persoanlly give people a hard time and try to educate others about this.

    1. The workplace is another one, an employee has the flu but is not granted sick leave, so what the business gets in those days is a miserable and groggy employee who spreads the virus around, as well as making a lot of mistakes that cost double in time and effort to detect and correct.  This is the rule and not the exception.

      That goes quadruple for workers in the food and services industry.  One example, there’s a popular fish taco stand in my town, I was suspiciously watching the girl in charge of the toppings, she had red eyes and a runny nose.  Suddenly, she sneezed right on the chopped cucumbers and pickled onions.

      How many people did she infect during her bout with the flu? I never did go back to that stand, but I’m sure this sort of thing happens with most food vendors, as duty seems to be more important than being a freaking public health hazard.

      1. I worked in a hospital and they used to give people shit about using their sick leave. It was messed up.

        1. You’re absolutely right, the health industry is the absolute worst place for this to happen, and now that you bring it up, few things have alarmed me more than a shift of coughing nurses, while my wife was suffering a severe bout of asthma.

          How many times have we heard of “complications” arising after surgery?  I’d be willing to bet that most of the time it’s pneumonia caught right there in the hospital.  Which may take your life, or at the very least extend an already horribly expensive hospital stay.

          It’s quite clear when framed in a certain way:  Where are the diseases concentrated?  In the hospital, of course.

  3. It’s fair to say that we don’t totally understand anything. Bertrand Russell suggested that the feeling of certainty was a bad position to occupy philosophically. And even when you do understand something fairly well, they have a devilish way of changing, diseases especially.

  4. “the simple story of one woman who infected thousands”… ahem, that would be one woman who infected 51 people. 
    “Typhoid Mary”, Mary Malone, was the victim of anti-immigrant (she was Irish) craziness – there were others who infected far more people. She ended her life in solitary confinement because she became a symbol of the dangers of immigrants.

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