-- intro: There's a new mystery surrounding a respiratory virus suspected of sickening children in 45 states since August: temporary paralysis.
"It's the wheezing you have to watch out for," said ABC News' chief health and medical editor, Dr. Richard Besser, referring to the whistling sound generated when air moves through narrowed breathing tubes.
Here are six things you should know about the outbreak.
quicklist: 1category: title: The CDC is Investigating the link between Enterovirus 68 and paralysis.url:text: Nine children in Denver have reported neurological symptoms after having a respiratory virus, according to the CDC, which says it is now investigating a link between enterovirus 68 and paralysis. The patients are all 18 years old and younger, and four of them have tested positive for enterovirus 68.
"It is a spectrum of arm or leg weakness that can be as mild as weakness or as severe as paralysis," said Dr. Larry Wolk, the chief medical officer and executive director for the Colorado Department of Public Health and Environment. "What ties them all together though are findings of spots or lesions in the grey matter of the spinal cord on MRI scans."
quicklist: 2category: title: This Isn’t the First Enterovirus 68 Outbreak in the U.S.url:text: Georgia and Pennsylvania reported clusters of enterovirus 68 almost exactly five years ago in September 2009, according to a 2011 CDC report. Arizona had a small cluster of cases in August and September 2010, according to the same report.
quicklist: 3category: title: No One Knows How It Startedurl:text: While this isn't the first time enterovirus 68 has popped up in the U.S., health officials are still trying to figure out why the virus has re-emerged.
"This is a very common time for outbreaks. Kids come back to school, they like to share things, they bring them home to their little brothers and sisters," said Besser, adding that most enterovirus outbreaks occur in the summer. "But this one, this particular enterovirus is very rare, and they have no idea why it showed up this year."
quicklist: 4category:title: No One Knows How It Spreadsurl:text: Studies on enterovirus 68 are limited, and so is knowledge about how the virus spreads.
Most enteroviruses spread through contact with respiratory secretions like saliva and mucous as well as feces, according to the CDC.
The Department of Health and Senior Services in Missouri, where hundreds of suspected cases have been reported, recommends washing hands thoroughly and often, avoiding close contact with people who are sick, disinfecting frequently-touched surfaces and staying home when feeling sick.
"The important thing is to recognize the signs of respiratory distress," said Besser, describing how difficulty talking, audible wheezing and bluish lip color can signal distress. "There are treatments to improve respiration."
Dr. Raju Meyappan, a pediatric critical care physician at Rocky Mountain Hospital for Children in Denver, said he's seen multiple asthmatic children end up on breathing tubes in the intensive care unit after contracting the virus.
"As a pediatric ICU doctor, we try our best not to intubate kids with asthma at any point in time," Meyappan said. "They all needed it. The onset [of the virus] is severe."
Children also appear to be more susceptible than adults, according to a CDC report released earlier this month about cases in Missouri and Illinois. The ages of those infected ranged from 6 weeks to 16 years, with most of the illnesses occurring in children aged 4 and 5.
ABC News' Dr. Richard Besser contributed to this story.