The virus that first appeared on August 18 has already sickened more than thousands of children in 10 American states. (Representational Image)U.S. Air Force photo by Tech. Sgt. Tony Tolley/ Naval Surface Warriors/Flickr

The US health authorities have issued warning against a mysterious respiratory illness that is spreading fast among American children.

The virus that first appeared on 18 August has already sickened more than thousands of children in 10 US states.

More than 900 cases have been reported in Colorado alone. North Carolina, Iowa, Illinois, Missouri, Ohio, Georgia, Kansas, Kentucky and Oklahoma are the other states affected by the disease, ABC News reported.

While the original culprit is yet to be traced, latest reports showed the role of human enterovirus 68 (EV-D68), a rare virus that was first isolated in 1962, in four California-based children with respiratory illness.

Though experts are yet to identify the reasons that led to the recent outbreak, doctors suspect school reopening to play a major role, CNN reported.

"We're in the middle of looking into this," Mark Pallansch, virologist and director of Division of Viral Diseases in CDC, told CNN. "We don't have all the answers yet."

"It could have taken off right after school started. Our students start back around August 17, and I think it blew up at that point," Dr. Mary Anne Jackson, division director for infectious diseases, at the Children's Mercy Hospital in Kansas City, told CNN. The hospital attended 475 cases, including 60 that needed intensive care. "Our peak appears to be between the 21st and the 30th of August. We've seen some leveling of cases at this point."

The disease that does not have a vaccine or specific treatment has already raised concerns among experts. "It is only 10 states now, but it's going to be across the country. So if your state doesn't have it now, watch for it, it's coming," ABC News Chief Health and Medical Editor Dr. Richard Besser, told the channel.

The virus strikes children under 12, particularly those who are asthmatic. The respiratory illness appears as a mild cold, but develops into a dangerous wheezing condition soon and does not respond to most of the common treatments available for asthma.

Common symptoms of this illness are rash, difficulty in breathing, fever and wheezing.

An online investigation shows that the virus, in some rare cases, can enter the central nervous system and cause paralysis or mortality.

In February this year, the same virus was linked to a polio-like illness that paralysed many US children, at least 25 in California. The disease appeared with severe breathing difficulties, but led to paralysis of limbs about two days after the symptoms started.