A mutated version of an adenovirus, a common family of viruses that normally causes simple infections, has caused severe respiratory illness in patients of all ages, including healthy young adults, say U.S. health officials.

The new and virulent strain of adenovirus serotype 14 (Ad14) killed 10 people in parts of the United States earlier this year, mostly from severe pneumonia. It also put dozens into hospitals, the U.S. Centers for Disease Control and Prevention reported in its Morbidity and Mortality Weekly Report on Thursday.

It also caused at least 140 illnesses in outbreaks in New York, Oregon, Washington and Texas. More than 50 of those patients were hospitalized, including 24 who were admitted to intensive care units.

Two of the 10 people who died from the new strain were infants. One was a 19-year-old female recruit at Lackland Air Force Base in Texas, where more than 100 other cases were found.

Most of the patients began their illnesses with symptoms of cough, fever or shortness of breath, which then developed into pneumonia.

"The cases described in this report are unusual because they suggest the emergence of a new and virulent Ad14 variant that has spread within the United States," according to the CDC report.

The cases occurred in 2006 and the first half of 2007; yet no cases have emerged since the spring.

What that means isn't clear, officials admitted.

"The fact that we haven't seen it and detected AD14 does not mean it's not circulating," Dr. Larry Anderson, director of the division of viral diseases in the national centre for immunization and respiratory diseases, said during a teleconference Thursday.

"I think it does mean, though, that it's not causing major clusters that present themselves as a public health threat."

No cases found in Canada

The Public Health Agency of Canada alerted health professionals in late April to the fact that clusters of disease caused by this new strain had been reported in the United States.

"Since we sent that note to physicians back in April, no outbreaks or cases of severe respiratory illness associated with adenovirus type 14 have been reported to the Public Health Agency of Canada," spokesperson Alain Desroches said in an email to CTV News.

CDC officials don't consider the mutation to be a cause for alarm for most people, and they're not recommending any new precautions for the general public.

"It's not a broad public health threat that we're seeing at this time,'' said Anderson.

There are more than 50 types of adenoviruses that can cause human illnesses. They are one cause of the common cold, and can also trigger pneumonia and bronchitis.

Dr. Neil Rau, an infectious disease specialist, says the fact the first cases occurred as long ago as May 2006 and we haven't seen more cases, is positive news.

"If it had been very easily transmitted person to person, we would have seen many, many more cases. And it would have spread to many more states and countries," Rau told CTV Newsnet in Toronto on Friday.

The concern, he said, is there is no treatment thus far and experts still don't know how the virus is spread. The CDC report is reassuring, however, in that health care workers who caught the virus did not get very sick.

"So it's not axiomatic that you are going to get really, really sick, unlike SARS, which seemed to have that effect," he said.

The viruses can also cause other illnesses, including pink eye, bladder infections and rashes. Colds caused by adenoviruses can be very severe in the very young and the very old as well as in certain other people, like those with compromised immune systems.

Yet other people infected with adenoviruses don't suffer symptoms at all, or simply come down with a common cold.

The Ad14 form of adenovirus was first identified in 1955. In 1969, it was blamed for a rash of illnesses in military recruits stationed in Europe. It's been detected rarely since then, but seems to growing more common.

There are no licensed medications for adenoviruses. Patients usually are treated with liquids, bed rest and pain relievers.