The number of people with the respiratory syncytial virus, or RSV, has surged, with cases hitting a two-year high, according to data by the Centers for Disease Control and Prevention.
RSV is twice as high among people of all ages at this point this year compared to last year, with the U.S. seeing more than twice as many cases per week, according to ABC News contributor Dr. John Brownstein, an epidemiologist and chief innovation officer at Boston Children's Hospital.
One reason for the surge amongst kids is because they are socializing in ways reminiscent of how they mingled pre-pandemic, Brownstein told ABC News.
MORE: Is the US facing a potential 'tripledemic' of flu, RSV and COVID-19?"Those respiratory viruses have been on a hiatus because kids have been somewhat isolated and have returned to normal school," he said.
While RSV affects children and adults, it's particularly dangerous for kids under 1-year-old and seniors 65 years old and older.
According to the CDC, there are between 100 to 500 pediatric deaths and 14,000 adult deaths each year related to RSV, with the actual figure likely being higher due to undercounting.
There isn't much testing for RSV because of the prevalence of the illness.
Pediatric hospitalizations from RSV are up 1% this week compared to last week. Nationally, pediatric bed occupancy is the highest it's been in 2 years -- with 76% of the estimated 40,000 beds filled with patients, according to the U.S. Department of Health and Human Services.
Washington, D.C. and 17 states, including Tennessee, Connecticut, Delaware, Arizona and Rhode Island, are seeing between 80% and 90% of pediatric hospitalizations bed capacity, according to HHS data. Maine sits at 102%, which means it has more patients than beds available.
Brownstein said that RSV isn't the sole cause of the hospitalizations among kids, as other respiratory diseases, including the common cold, are spreading.
The rise in RSV cases on top of the spread of COVID-19 and the flu during the fall and winter have doctors worried that Americans are facing a potential "tripledemic" this year.
According to the CDC, respiratory illnesses have appeared earlier and in more people than in recent years. The federal health agency says there have also been early increases in flu activity across most of the U.S., with indications that this season could be much more severe than the recent seasons.
ABC News' Eric Strauss contributed to this report.