CHICAGO - Metapneumovirus, discovered just two years ago, is turning out to be an exceedingly common cause of human misery, responsible for garden-variety colds in grownups and more severe coughing, wheezing and congestion in children.
Researchers are beginning to piece together the scope of this ubiquitous but overlooked virus, which appears to afflict just about everybody, probably over and over.
Even though the bug rarely seems to be serious, its vast presence intrigues microbiologists, and it is one of the most talked-about topics at this week's meeting in Chicago of the American Society for Microbiology.
Experts say the metapneumovirus is almost certainly not a new bug but something that has been around for eons.
Like many other respiratory bugs, this one is most likely to cause severe illness in infants encountering it for the first time. Although repeat infections are thought to be common, they result in much less intense illness, often just an ordinary cold or perhaps no symptoms at all. However, the bug might cause more serious problems in the elderly and people with other medical conditions.
Although new microbes, such as the ones that cause AIDS and SARS, are occasionally recognized, it is unusual to discover a virus as omnipresent as this.
"Thousands of hospitalizations occur every year due to this virus in infants," said Dr. James Crowe Jr. of Vanderbilt University.
His research suggests the metapneumovirus is second only to respiratory syncytial virus as a cause of severe lower respiratory infections in the young, occurring about two-thirds as often. Both viruses are members of the paramyxovirus family.
Crowe's team looked at nasal specimens taken from 2,000 children after they were treated for lower respiratory infections since 1976.
The newly discovered virus turned out to cause about 12 percent of these severe illnesses. They also caused 15 percent of common colds in children, including one-third of the colds complicated by middle-ear infections.
"When we put this in perspective, it appears to be the second-most common cause of respiratory illnesses in children," he said.
The virus went undiscovered because it does not grow well in cell cultures, a standard tool for sorting out the viruses that cause human disease. It was identified in 2001 by researchers from Erasmus Medical Center in Rotterdam. They calculated every child catches the virus by age 5.
As with many respiratory viruses, such as the rhinoviruses and coronaviruses that cause many colds, the body does not remember the metapneumovirus clearly and so contracts it over and over, even though repeat bouts seem to be milder.
Now that scientists know what to look for, they can seek out the virus' genetic footprints in nasal secretions. This way, they are trying to assess how often, and how severely, it makes people sick through life.
"At least in adults, we are figuring out whether it is a big deal. It is a little too early to say," said Dr. Ann Falsey of the University of Rochester.