DENVER (AP) _ A rare, virulent form of strep is on the rise in at least six states and has been blamed for 11 Denver-area deaths, investigators trying to determine who else is at risk said Thursday.

Another 29 cases have been reported in Denver in the last six months. The fatality rate is about 25 percent, health officials say.

Investigations were under way in parts of Colorado, Alabama, Arizona, California, Maryland and Ohio, where severe strep clusters have been reported. Since Sept. 1, 40 severe strep case have been reported in those states, said Dr. Ben Schwartz, an epidemiologist in the CDC's respiratory disease branch in Atlanta.

Forty severe strep cases have been reported since Sept. 1, Schwartz said.

''The strep bacteria has been around a million years and we're learning more about it to prevent people from getting it. It's actually rather mundane,'' Dr. Roger Gollub, epidemic intelligence service officer for the Colorado Department of Health, said in Denver.

The outbreak ''has nothing to do with (common) strep throat,'' Gollub said. ''It's just the strep organism is involved.'' All the victims were elderly and suffering from other illnesses, too, Gollub said.

Common strep symptoms are a sore throat, swollen glands and fever. They can be treated with penicillin.

The severe form of strep is characterized by a sudden onset of a very high fever, chills, shock and low blood pressure. There also may be skin or muscle infections and pneumonia, the investigators said.

The severe form, known as group A streptococcus bacteremia, gets past the body's primary defenses and enters the bloodstream through injury, infection from a surgical incision, or other ways that are still unclear.

Much of the information on the disease that's been collected by the U.S. Centers for Disease Control has come from Denver, said Schwartz.

Nationally, cases of severe strep have occurred at a rate of five per 10,000 population per year.

Health officials around the country ''have expressed interest in working with us and we think it gives a good geographical, rural-urban mix to the investigation,'' Schwartz said. ''It's not because there have been any problems in those areas; it just gives us a better picture.''

Health officials started investigating the bacteria a year ago when a cluster of cases was noticed in Denver.

''A year ago a very smart, perceptive infection control nurse at a local hospital noticed nine cases in a six-week period and since then we have been looking into it,'' Gollub said.

Schwartz said he did not want people getting the impression that they can die from strep throat or that the disease is rampant.

''The important thing to realize is that strep throat is very common, all kids get it and this is actually strep throat season,'' he said. ''Parents should really not be worrying that it is going to be turning into the severe disease.''

But, Schwartz said, if a parent believes that his child has strep throat, it should be diagnosed and treated.

A Denver doctor is working with the World Health Organization to investigate a possible link with a reported strep outbreak in Europe. Schwartz said the CDC also is in contact with European health officials and has heard of some problems in Scandinavia and England.