It's not polio—but news of five cases of a rare polio-like disease that has left five California children with one or more paralyzed limbs has riveted the public and stirred up worries and confusion. The children, all of whom had been successfully immunized against polio, range in age from 2 to 16, and their paralysis—which came on suddenly, as if out of nowhere—is apparently incurable.

The California Department of Public Health is also investigating 20 more reports of the syndrome, but according to a statement released earlier this week, "Thus far, the Department has not identified any common causes that suggest that the cases are linked."

Are these cases a tragic anomaly or a harbinger of a public health threat? "We're not considering this an outbreak," says Jane Seward, the Center for Disease Control and Prevention's deputy director for the division of viral diseases. A small number of cases of acute flaccid paralysis (or AFP, sudden-onset weakness or paralysis) occur every year around the country, and the current numbers are "well within the expected range," she explains.

"The CDC expresses care and concern for the children" and their families and is working closely with the California Department of Public Health, she says. "But at this time we don't think the situation in California is a cause for public concern."

Putting What We Know—and Don't Know—in Perspective

Yet despite these reassurances, questions persist, as does a sense of uncertainty about how to place these cases in perspective. Certainly, whenever the word "polio" appears in a health story, fears are aroused—and understandably so, given that until the 1940s and '50s, polio epidemics around the country disabled approximately 35,000 people each year, mostly children. Thanks to the development of the antipolio vaccine in the 1950s, the United States (and most of the world) is now polio free.

But polio still exists in three countries: Afghanistan, Nigeria, and Pakistan. And outbreaks can occur. It's therefore important, when cases of AFP appear, to test for polio as well as to identify the virus responsible, says Diane Griffin of Johns Hopkins' Bloomberg School of Public Health in Baltimore.

The cases in California are not polio. But viruses other than polio can cause this kind of paralysis, she says. "They just do it much less frequently than the polio virus does." That means the cases being seen now in California, while both extremely infrequent and enormously sad, do occur. In this case, she says, "people are looking [to see] if there is some unusual increase [in paralysis] due to a particular virus."

Are the California cases evidence that such paralysis is indeed occurring a little more frequently than usual? And is there a single virus that is responsible?

Those are the questions that neurologists Keith Van Haren of Lucile Packard Children's Hospital at Stanford University and Emmanuelle Waubant of University of California San Francisco Medical Center asked after they encountered the five AFP cases in their hospitals over the course of a year. "Normally, you'd expect to see this kind of case once every five years," says Van Haren. To put it in perspective, this is "a modest increase of a very rare syndrome," he says.

But it nonetheless seemed unusual and warranted pursuing. They wondered if what they were seeing was "simply statistical noise," essentially a coincidence of timing, or "whether this is more significant," he says.

Is EV68 to Blame?

Waubant emphasizes that the five cases are not clustered around a particular place: They occurred within a 100-mile radius. And there are no common factors beyond the presentation of their symptoms and their outcomes. Two of the five children, however, did test positive for enterovirus-68 (EV68).

Both the polio virus and EV68 are among the more than 200 related viruses in the Picornaviridae family. Like polio, the enterovirus is spread by person-to-person contact, but it generally leads to nothing more serious than a common cold, if that. But in rare instances, enterovirus infections have caused meningitis, encephalitis—and paralysis. Indeed, over the past decade EV68 has been associated with small numbers of AFP cases in Asia, Europe, and the United States.

So, is EV68 the culprit? "We still don't know," says Waubant. Three of the five patients were not seen by Waubant and Van Haren until several weeks after the onset of paralysis, by which time their systems would have already cleared the virus that was responsible, she says. As for the other two, it could be coincidental that they found EV68. Or, says Van Haren, "it may be that we're seeing a couple of different viruses causing these cases."

Wondering if other physicians were observing any similar cases, Van Haren and Waubant wrote their report and submitted it to the American Academy of Neurology, whose press release triggered an immediate and overwhelming media response. The bottom line, according to Waubant, is that "we don't think that there is an epidemic about to hit. What we're dealing with is extremely rare."

To be sure, says Waubant, in the sudden onset of acute weakness, it's essential to consult your primary health care provider immediately. But, she says, "you should not panic if your kids have a cold."

What the Public Needs to Know

Key questions do remain: Is it mere chance that this rarely seen syndrome has had a slight uptick of late? Is this a new virus or a new viral strain that may prove more likely to cause severe outcomes than previous strains?

To help determine the answers, "we want other physicians to be aware that we've seen these cases, and encourage them to contact the Department of Health if they see cases like this," says Van Haren. "We're seeing this as a modestly increased rate than we would expect in our practice, and we'd like other practitioners and medical personnel to be on the lookout" and, if they see similar cases, to provide their patients' blood, spinal fluid, stool, nasal, or mouth swabs for testing.

In the meantime, says Van Haren, "this should not alter any aspect of anyone's family life or child care. What we hope it will do is alter the referral pattern among specialists so they can send samples to the Department of Public Health and help us determine if this is a phenomenon that is increasing above baseline and what is causing it."

The need for answers can be seen in the five children who have been left partially paralyzed.