The Washington University School of Medicine researchers, led by David Wang, said clinicians can typically use a patient's symptoms to determine a virus is the likely culprit in a respiratory infection. But, even with advanced testing, they can't pin the blame on a particular virus in roughly one-third of all such infections.

Scientists can't yet prove the new virus, known as the WU virus, is making patients sick. But Wang has started follow-up studies.

"We've completed the first step required to link the WU virus to disease," said Wang, an assistant professor of molecular microbiology. "First, you have to detect the potential pathogen in someone who's sick. Then you have to develop a way to grow the new micro-organism in the laboratory. Finally, you have to show that you can make an animal model sick by exposing it to the micro-organism."

The research appeared in journal PLoS Pathogens.