A virus-caused respiratory disease that has already sent hundreds of children -- its apparent main target -- to hospital emergency rooms and put some in intensive care units is likely to spread even farther, officials warn.

Known as Enterovirus 68 or EVD-68, it's been confirmed in 153 cases in 18 states, according to The Centers for Disease Control and Prevention. 

Enteroviruses are a common type of virus -- there are more than 100 varieties, one of which can bring on the common cold -- but the serious impact of EVD-68 has caught even health officials off guard.

EVD-68 is thus named because it is the 68th enterovirus discovered.

Although it has been known to doctors since it was first identified in California in 1962, it has generally only occurred in small clusters around the United States -- until this year's widespread outbreak.

Through 2005, the CDC had received only 26 confirmed reports of EVD-68 infections.

In the current outbreak, cases have now been confirmed in New Jersey, New York, Pennsylvania, Illinois, Iowa, Indiana, Missouri, Nebraska, Alabama, Connecticut, Kansas, Louisiana, Kentucky, Oklahoma, Minnesota, Colorado, Montana and Virginia.

Most enterovirus infections begin with symptoms such as fever, coughing and a runny nose or sneezing, but the symptoms of EVD-68 have differed from that normal pattern, says Dr. Mary Anne Jackson, head of the infectious disease division at Children's Mercy Hospital in Kansas City, MO, where the initial cases in the current outbreak were recognized.

"Only 25 percent to 30 percent of our kids have fever, so the vast majority don't," Jackson says.

Instead, she says, they develop a severe cough that can lead to trouble with their breathing.

That's often accompanied by wheezing, as if they were suffering from asthma, even when there's no history of it in the child, she adds.

"They're just not moving air," she explains.

As to why children seem to be the main victims of the virus, doctors suggest symptoms in adults may not be as severe and adults may get better without needing treatment so cases are not being recorded, although some adult cases of EVD-68 have been confirmed.

The rapid spread of the disease is typical of many such viruses, which can be spread through sneezing, coughing or touching infected persons or surfaces.

There's a normal "season" for enterovirus infections that runs from July into October, and with many U.S. children returning to school at the end of August and through September the opportunities for such diseases to spread among them is increased, doctors say.

Many hospitals reported an increase in cases just as schools reopened.

EVD-68's previous rarity may be one reason it's spread so quickly in the current outbreak, Dr. Jackon says.

"If you have a new virus that has not widely circulated, most people are going to be susceptible," she says.

The situation could worsen before it starts to abate, she cautions.

"In order for this virus to stop, it's going to have to infect enough of the population to provide immunity and essentially burn itself out," she says.

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