"We've added a bunch of viruses, some of which we have never heard of before," said Kenneth McIntosh of Harvard Medical School, speaking at a microbiology meeting in Chicago.

Many of these newly named viruses have been causing coughs and runny noses for hundreds of years, alongside better-known viruses such as rhinoviruses and adenoviruses.

But scientists have only recently had the molecular research tools to identify these bugs, McIntosh said.

"We know they are there ... but we don't know what we really need to know about their capacity to produce disease," he said.

That information may prove to be important as cold viruses are the main culprit behind 50 percent to 80 percent of asthma attacks.

"In children, up to 80 percent of the time when you have a bad asthma attack, it's because you've got one of these wimpy cold viruses," said Dr. Jim Gern of the University of Wisconsin School of Medicine and Public Health.

"With the advent of new tests that are based on genetic material, we're finding there's a lot more out there that we were not able to detect before," he said in a telephone interview.

SARS CATALYST

Some of the research activity was spurred by the outbreak of severe acute respiratory syndrome or SARS, which spread from China to 30 countries in 2003, infecting close to 8,000 people and killing nearly 800 before it was contained.

"After SARS turned out to be a coronavirus, that energized the coronavirus field, so we've added two new coronaviruses," McIntosh said.

Another new virus is the human bocavirus, a close cousin to the bovine parvovirus and the canine minute virus, which cause diarrhea in cattle and dogs. "Now it is being found in acute respiratory disease in children," McIntosh said.

The other puzzling virus is the mimivirus, first discovered growing inside an amoeba in 1992 but which evaded identification until 2003 because of its enormous size and complex characteristics.

Three times bigger than other viruses, the mimivirus was found in the DNA of patients with pneumonia, and may account for some of the 20 to 50 percent of pneumonia cases that previously went unidentified.

Last May, David Wang of Washington University School of Medicine in St. Louis discovered the WU virus, a new type of polyoma virus that may be causing respiratory infections and perhaps other problems.

"One of the things we are looking at is does our virus have the ability to create tumors," Wang said.

Gern, who wrote about the new molecular sleuthing techniques in the September 15 Journal of Infectious Diseases, said his group and others are focusing on why cold viruses make some people very sick and not others. That may lead to therapies that strengthen the immune system.

"I think the most promise lies in the area of finding immune responses that seem to be protective against these illnesses," Gern said.