Health experts warn that the tridemic will continue past holiday season

(CNN) - Public health experts fear a surge in respiratory viruses following holiday gatherings and New Year's Eve celebrations.

They're concerned about three viruses in particular: flu, COVID-19, and Respiratory Syncytial Virus (RSV).

Holiday gatherings provide extra opportunities for these illnesses to spread, and cases did surge after Thanksgiving.

Another factor is the wave of flight cancelations sweeping the country.

Large amounts of people, stuck in crowded, stressful settings at airports, appears to be a recipe for viral spread.

The CDC says seasonal flu activity remains high in the U.S., but continues to decline in most parts of the country.

COVID-19 increases appear to be relatively mild.