This is how outbreaks occur. It isn’t complicated.

I’ve read way too much reporting, and way too many tweets, parsing how the latest measles outbreaks happened. Some are missing the point. This is how outbreaks of a disease that isn’t endemic to the United States occur:

  1. Someone travelling/living abroad contracts the disease and comes to the US
  2. Other people who are susceptible to the disease come into contact with them here at home
  3. Those people contract the disease
  4. Go back to step 2

That’s it. It doesn’t matter if the person from step one was an illegal immigrant, a doctor working overseas, or an Amish Missionary. Since we can’t control what other countries do, and we live in a world where people travel, (1) is going to occur at some point.

What we CAN do is try to prevent other people here from getting the disease. That’s where vaccination comes in. If everyone is vaccinated against measles, for instance, than – yes – a very small number of people might contract the disease when (2) occurs, but the vast, vast majority of people who come into contact with the infected person will be fine.

The system breaks down, and outbreaks occur, when more people are susceptible. Everyone, for instance, is susceptible to Ebola at a certain point in the illness. So we have to be careful to quarantine people who are infected when they are sick. But Ebola is relatively hard to catch. It has an R nought of 2, meaning that an infected individual might infect, on average, 2 others. But measles has an R nought of 18. It’s one of the most infectious pathogens around.

Quarantining is difficult, if not impossible. The virus is unbelievable hardy and easy to catch. So the absolutely, positively best thing you can do it to be vaccinated. Period.

I should point out that it also doesn’t matter to the outbreak why people remain unvaccinated and susceptible. It can be because of religious reasons. It can be because of irrational fear. It can be because they’re “hippies”. I don’t care – the outbreak is the same. (1) is going to happen. But if everyone was vaccinated, then the infected person wouldn’t make national news because it would be very hard for it to go much beyond themselves.

The important part of stopping an outbreak of measles isn’t (1). That’s going to happen every once in a while. The important part is that too many people in the United States remain unvaccinated and susceptible to measles for any number of reasons. That’s what’s “causing” the outbreak. That’s what we need to focus on. Full stop.


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