When a mysterious disease fells people - as happened in California recently, with as many as 20 children experiencing unexplained paralysis - teams of physicians and epidemiologists quickly mobilize. Perhaps you saw the movie "Contagion"? The idea is to find the culprit before it spreads but also to prevent public panic.
The investigation typically begins with a doctor reporting a sudden increase in patients with a particular disease or symptom to a state health department. It then falls to the government to determine whether the report is a false perception, a statistical quirk or a genuine surge.
The paralysis cases are a classic example: The symptoms are well known but their incidence appears to have spiked. Acute flaccid paralysis, the technical term for the symptoms observed in California, is something that many pediatricians have seen, and it has myriad causes. "I probably see one case like this every five years," says Keith Van Haren, the Stanford neurologist who is leading the hunt for an explanation for the paralysis reports. "Five cases in one year seems like an abnormality."
It's much harder to prove an abnormality than to perceive one, though. Surveys by the World Health Organization suggest that approximately one of every 100,000 children each year develops acute flaccid paralysis. It's not clear whether the rate is the same within the United States, but, since approximately 7.7 million kids younger than 15 live in California, 20 cases in a year might be within a normal range. A final determination on that question, however, will depend on whether the cases are related and whether they are clustered within a small geographic range. There is also a risk that clusters of cases can generate more such reports, especially once the media get involved.
"What you start looking for, you see," says Daniel Firkin, epidemiology branch chief in the division of viral diseases at the Centers for Disease Control and Prevention. Physicians usually don't report childhood paralysis, he explains, because the CDC doesn't require it.Now that they're on notice, however, more cases are going to be reported. It can be difficult to distinguish a surge in cases from a mere surge in reports.