Q. A 6-month-old daughter of a friend of mine recently had to be hospitalized for an infection I had never heard of -- respiratory syncytial virus. Is this some kind of new infection that attacks children? How common is it? What kind of symptoms does it cause? Is there any treatment for it, or does it just have to run its course like most viral infections? A. Although its name may not be familiar to most parents, respiratory syncytial virus, or RSV, is the major cause of lung infections in infants and young children. At these ages, RSV causes nearly half the cases of pneumonia and most cases of an asthma-like illness known as bronchiolitis.

Bronchiolitis is a lung infection that starts like a bad cold but quickly progresses to a sometimes frightening case of difficult or rapid breathing and wheezing.

RSV is found all over the world. In the United States, outbreaks strike in the late fall, winter and spring months. RSV is so common that most children have been exposed to it by the time they are 2 years old. Not all exposures lead to serious illness, however, and many children and adults build up resistance to the effects of RSV infection.

Young children are especially prone to serious infection, which most often strikes infants under 6 months of age. Severe illness usually requires treatment in the hospital. Fortunately, a relatively new medicine helps combat this potentially deadly virus.

Until recently, we didn't have drugs to fight most viruses. Research breakthroughs have recently lead to a handful of new anti-viral drugs, including one against RSV. Called ribavirin (Virazole), this new medicine must be given by inhalation treatment in the hospital. Unfortunately, there are no pill or injection forms of this drug available. For this reason, ribavirin is used only in hospitalized children with severe cases of RSV infection.

How do you catch RSV? Like most cold viruses, RSV is passed from person to person by coughing or hand-to-hand contact that passes invisible amounts of virus-packed nasal secretions. People wipe their runny nose and then pass on hordes of infectious viruses with the next handshake or touch.

RSV spreads rapidly through families and day-care centers. Most people without immunity to this infection will come down with it. Compared to children from smoke-free homes, children living with a parent who smokes are more likely to develop an illness from lung infection.

Besides causing colds, ear infection and pneumonia in children, RSV is a common cause of colds in adults. RSV colds are often worse and last longer than the typical common cold. They can be flu-like, with fever, muscle aches, headache, fatigue and bronchitis.

RSV infection can lead to some worrisome complications. Children who have had bronchiolitis are more prone to suffer from asthma and slightly reduced lung function later in life. However, some evidence suggests that these children have altered lung function to begin with, making them vulnerable to troublesome RSV infection.

The elderly are also subject to the ravages of this common virus. In closed communities, such as nursing homes, death rates due to RSV infection have been as high as 50 percent.

Doctors diagnose RSV by testing for the presence of this virus in nasal secretions or sputum from the lung. This test is usually done only in sick children or in those for whom treatment with ribavirin is being considered. RSV testing is not normally done in older children and adults with respiratory infections.

RSV spreads quickly to susceptible people, so it can be difficult to prevent infection from striking. Washing your hands after coming into contact with a potential carrier is probably the best way of preventing spread. No vaccine against RSV is available.

Jay Siwek, a family physician from Georgetown University, practices at the Fort Lincoln Family Medicine Center and Providence Hospital in Northeast Washington.