A virus could be adding to the seasonal misery brought on by influenza, but it's not being identified, an infectious disease specialist told NBC News.
Adenovirus can cause severe flu-like symptoms, and the U.S. military already vaccinates recruits against two of its 52 strains.
But most people don't get the vaccine and aren't tested for it at doctor's offices, according to Adriana Kajon, the specialist at the Lovelace Respiratory Research Institute in Albuquerque.
"We are seeing severe adult infections," Kajon said. "That's a big deal, especially for a disease that by all means is vaccine preventable. But this vaccine is not licensed to be used in civilians."