Runny nose, fever, cough, even pneumonia -- the symptoms sound like swine flu but children hospitalized at one U.S. hospital in fact had a rhinovirus, better known as a common cold virus, doctors said on Tuesday.

Hundreds of children treated at Children's Hospital of Philadelphia had a rhinovirus, and federal health investigators are trying to find out if it was a new strain, and if this is going on elsewhere in the country.

What began to happen in early September is we started seeing more children coming to our emergency room with significant respiratory illness, said Dr. Susan Coffin, medical director of infection control and prevention at the hospital.

Doctors and parents assumed it was the new pandemic H1N1 swine flu, which would be expected to re-emerge as schools began in September. But it was not, Coffin said in a telephone interview.

The hospital, unlike most hospitals in the United States, runs a test that can diagnose 10 different respiratory viruses, including influenza but also rhinoviruses, parainfluenza viruses and other germs that make kids sick.

The data showed us it wasn't H1N1 but instead was this rhinovirus infection, Coffin said.

Usually rhinoviruses cause an annoying but benign illness that looks a lot like flu, but with more runny nose and usually less of a fever. This one was causing severe symptoms and even pneumonia.

Some of these kids had really bad wheezing, Coffin said -- so bad they had to be hospitalized and treated with a nebulizer, which delivers drugs into the lungs to help keep oxygen in the blood.

We don't terribly often have large numbers of children test positive for it, Coffin said.

CDC INVESTIGATING

But she estimated that 500 were hospitalized in September and October, with no deaths that she knows of. Starting in mid-October, H1N1 swine flu started to show up, too.

The U.S. Centers For Disease Control and Prevention is investigating, said CDC spokesman Dave Daigle.

While rhinovirus outbreaks are common in the fall, the outbreak that occurred this year was unusually large and resulted in a lot of hospital admissions, including many children that required intensive care, Daigle said.

We're still testing the strains from the outbreak, but from what we've seen so far, it doesn't appear that there's a single predominant strain.

The CDC says while swine flu is above epidemic levels, only 30 percent of cases of so-called influenza-like illness that are tested actually turn out to be H1N1.

Coffin and CDC officials say it is important for people not to assume if they or their children have flu-like symptoms that it wasswine flu and that they do not need to be vaccinated.

H1N1 has infected an estimated 22 million people and killed 3,900 in the United States alone. It continues to spread globally and governments are just at the beginning of efforts to vaccinate people against the virus.

There is no vaccine for rhinovirus and no good treatment. For severely ill patients hospitals can try to keep blood oxygen levels up and keep the patients hydrated, often with intravenous lines if they are coughing or wheezing too hard to eat or drink.

Medicine