Health officials suspect that a rare respiratory virus is the reason that hundreds of children across America have been sickened in recent weeks.

The Fox News Medical A-Team's Dr. Marc Siegel gave his advice this morning, saying that the most important thing is for parents of children with asthma to be especially watchful.

The CDC believes asthma, combined with this illness, is causing kids to need hospitalization. No deaths have been reported.

Siegel said if your child has cold symptoms and/or a fever and is also having trouble breathing, bring the child to a doctor immediately.

He said it's very difficult to stop these illnesses from spreading in schools, and predicts it will spread nationwide.

Watch his analysis above and read more below from FoxNews.com:

The Denver Post reported that the Centers for Disease Control and Prevention believe that human enterovirus 68 is at the root of the epidemic, though testing of samples has not produced a definitive answer. 

The Post reported that officials at Children's Hospital Colorado have treated more than 900 children for severe respiratory illnesses since August 18, with 86 admitted to the hospital. At Rocky Mountain Hospital for Children, doctors put five children on a ventilator this week, including a 13-year-old boy whose condition deteriorated from a mild cold to a life-threatening illness overnight. 

"He was OK. Then he was unconscious. It was unreal," Jennifer Cornejo told the paper about her son William. "I thought my heart would come out of my chest. It was so horrible."

The illness appears to be most common among very young children and children who have asthma. The CDC tells ABC News that similar suspected outbreaks have been reported in nine other states -- Missouri, Kansas, Illinois, Kentucky, Iowa, Ohio, Oklahoma, North Carolina, and Georgia. The Columbus Dispatch reported that Nationwide Children's Hospital in Ohio's capital reported seeing an average of 73 patients with respiratory complaints per day between August 31 and September 2, a 40 percent increase. 

Doctors recommend taking basic sanitary precautions, including washing hands often with soap and water, avoiding sharing items with sick people, and disinfecting frequently touched surfaces.