The adenovirus has many of the flu's telltale symptoms but is rarely tested by doctors, leaving patients miserably sick until it passes.
Like the flu, the adenovirus can lead to many other illnesses, like upper respiratory infections, pneumonia and bronchitis. But, since there are 52 strains of the virus, it can also lead to other complications like gastrointestinal illnesses, urinary tract infections and pink eye, Live Science reported.
Symptoms of the virus include fever, sore throat, runny or stuffy nose and a cough, according to the Centers for Disease Control and Prevention, making it difficult to discern from the flu.
"It would be really hard to tell it apart from influenza," Johns Hopkins Center for Health Security senior scholar Dr. Amesh Adalja told Live Science.
But adenovirus has a "striking seasonality," Dr. Adalja said, occurring mostly in late winter, spring and summer, as opposed to the flu which spikes during the winter but can occur year-round.
Like the flu, adenovirus can be deadly for particularly vulnerable members of society, like the young, elderly and those with compromised immune systems, if to a lesser degree. An outbreak in the U.S. in 2007 made about 140 people sick across four states and killed 10, according to the CDC. By comparison, the CDC stated that the flu can cause between 12,000 and 56,000 deaths per year.
A vaccine for two more common strains of the virus has been administered to members of the U.S. military since 2011 because adenovirus spreads quickly and easily in close quarters, according to Medscape. It's estimated that about 15,000 cases of the disease are prevented each year in the military thanks to the vaccine, the U.S. Army Medical Material Development Activity reported.
The vaccine is currently being considered for people in similarly tight living situations like college students and long-term care residents, Adalja said.