Most people have by now seen the news about a mysterious "polio-like" illness paralyzing children across the US. To date, the CDC has confirmed 62 cases of the condition, called acute flaccid myelitis (AFM), in 22 states since August. Another 65 people are suspected to have it, but haven't had the diagnosis officially confirmed yet. 90% of the patients are under 18; the median age is 4. All of the cases have so far tested negative for poliovirus.

So what's going on? The headlines, at least, aren't too far off base. The term "polio-like" is pretty accurate here. A person with AFM loses the use of one or more limbs suddenly, for no obvious reason. They just go limp. Some recover, regaining full or partial use of the paralyzed limbs, while others don't. That's exactly what happens in paralytic poliomyelitis.

There are good reasons to suspect that a similar virus might be involved, too. The first documented outbreak of AFM in the US was in 2014, with a total of 120 confirmed cases in 34 states between August and December. There was a strong - but not absolute - correlation between those cases and enterovirus D68 infection. EV-D68 is a picornavirus, the same viral family that includes poliovirus. Some of the current cases have also tested positive for EV-D68, but the correlation seems a lot weaker this time.

One striking aspect of these outbreaks is that they're cyclical. The CDC data show this quite clearly, with distinct spikes in AFM incidence every other autumn since 2014. There's a more or less constant trickle of cases between peaks too.

And that's pretty much all the hard data we have. Beyond that we're left with educated guesses about what's going on. Here are mine.

First, I'd be surprised if periodic AFM outbreaks are a new phenomenon. The CDC didn't track non-polio AFM before 2014, so it's entirely possible that this cycle of outbreaks has been happening for a long time without anyone realizing it. While the individual cases are undoubtedly devastating to the families involved, the actual numbers are small in national terms, and the cycle wouldn't have been obvious. Sometimes a kid becomes paralyzed for no obvious reason. Sometimes they get better, sometimes not. Until the CDC started keeping track, it was just one more medical mystery.

Second, the pattern here strikes me as very picornaviral. Before vaccines drove it to the brink of extinction, poliovirus caused regular seasonal outbreaks. The virus spreads very easily, especially among children, and in an unvaccinated population nearly 100% will become infected. 99% of them will develop no symptoms worse than mild diarrhea. However, even 1% of a population becoming paralyzed is an immense social trauma; in the pre-vaccine era, everybody knew someone who'd been put in a wheelchair or iron lung by polio.

Now let's look at those two things together. Something, perhaps a virus, has likely been causing small, cyclical outbreaks of AFM every couple of years for a long time. During much of history, this wasn't noticeable against the background of annual polio epidemics. Indeed, a handful of people paralyzed by "polio" might have had some other disease, but the available tests at the time weren't specific enough to distinguish it, and in any case the treatment was the same. Now that we've virtually eliminated poliovirus, and ramped up surveillance for AFM, we could be seeing a signal that was there all the time.

The European CDC calls enteroviruses "the most prevalent viruses in the world." I think that's a stretch, but they may be the most prevalent among known human viruses. Poliovirus and EV-D68 have dozens of siblings, and I doubt you'll find a virologist who believes we've found them all. These are extremely successful viruses that everyone has been exposed to. Most of the time they cause no lasting harm. In the case of poliovirus, the infection can go awry and lead to paralysis. Maybe something similar can happen with some of its relatives.

At the very tip of this speculative limb, it's interesting that we have multiple disease processes apparently leading to the same outcome. That suggests there could be some common mechanism underlying poliomyelitis and non-polio AFM. Perhaps it involves conditions in the gut allowing an enterovirus to get into the nervous system in a minority of patients. With poliovirus we solved the problem by developing vaccines that prevented the infection. That's not practical with the rest of the enteroviruses; there are just too many of them to vaccinate against. Instead, we should focus on working out what drives these viruses down the nerve-damaging route, and attack that mechanism.

In the meantime, the interventions will sound very familiar: wash your hands, cover your cough, don't drink untreated water, and get vaccinated against the diseases we can prevent. AFM is dramatic and certainly scary, but we should really be more worried about the common stuff.

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