In most parts of the United States, RSV outbreaks usually occur during the winter, although there can be significant differences based on region. For example, the RSV season starts earlier and lasts longer in Florida than in other areas of the country, according to the report in the Dec. 7 issue of the CDC's Morbidity and Mortality Weekly Report.

During the 2006-07 season, labs in 36 states reported 21,470 infections.

Although the national RSV season hasn't fully started yet, the reports showed that infections were seen during the week ending Nov. 17 in the South and during the week ending Nov. 24 in the Northeast. Labs in the Midwest and West have not reported any RSV activity in November, the CDC said.

There is no vaccine for the virus, and most children recover in eight to 15 days, the agency noted.

"As is true of most respiratory viral infections, RSV is spread from person to person quite readily," said Dr. David Katz, director of the Yale University School of Medicine's Prevention Research Center. "Transmission can be reduced considerably by simple measures, such as routine hand-washing, and avoiding close contact with those infected."

The virus can be transmitted through droplets from sneezes and coughs, and from household surfaces on which the virus can lie.

For people most at risk of RSV infection, there is some protection available in the form of an antibody to the virus that is given monthly along with the antiviral drug ribavirin, Katz said.

"Good surveillance data enhance this effort by providing an early and reliable alert system for likely exposures," he said. "Armed with this information, pediatricians can make optimal use of the antibody against RSV. This is a clear case where forewarned is forearmed."

More information

To learn more about respiratory syncytial virus, visit the CDC.