There's one silver lining in the ominous news about the mysterious respiratory virus that has sickened children in 46 states since August, even causing paralysis in some: It does not seem to be spreading into adults.
But experts aren't sure why.
"Everyone is scratching their heads on this one," said Dr. William Schaffner, chairman of the department of medicine at Vanderbilt University in Nashville.
One possible theory is that the current pathogen, enterovirus 68, or a virus very similar to it has circulated undetected in the past, Schaffner said. That means adults may have already been exposed to it and have built up immunity.
However, Schaffner said it's more likely that adults aren't being infected because enteroviruses are so common.
"Older family members may have built up some antibodies to enteroviruses in general that are providing some cross-protection," he said.
Dr. Michael Tosi, chief of pediatric infectious diseases at Mount Sinai Hospital in New York City, said he doesn't expect to see many adult cases if any at all.
"In general we see enteroviruses more in children than adults anyway," Tosi said. "When they do get them they are often asymptomatic or have less serious reactions that don't require hospitalization."
Schaffner agreed it's unlikely there will be mass outbreak in older people. The virus has been around since the summer and children have been exposing their families to it for months, he said. If it was going to spread to adults, it probably would have already.
However, he noted that enterovirus 68 is a bit of a rogue.
"Many enteroviruses are transmitted and live in intestinal tract but this virus is spread by a respiratory route, more like how winter flus are spread," he explained. "We'll definitely have to watch it and see how it behaves."
Enterovirus 68 is similar to the common cold, but symptoms can be more serious, according to the U.S. Centers for Disease Control and Prevention. It causes wheezing and in some instances, neurological symptoms and temporary paralysis. How it spreads is unclear, though most enteroviruses spread through contact with respiratory secretions like saliva and mucous, as well as feces.