Get breaking news alerts and special reports. The news and stories that matter, delivered weekday mornings.

It looks like flu. It feels like flu. It even puts patients into the hospital like flu can.

There’s another virus out there that could be adding to the seasonal misery, but it’s not being identified.

The virus is called adenovirus, and it can cause very severe flu-like symptoms. It’s so risky that the U.S. military vaccinates recruits against two major strains.

But most people are not vaccinated against adenovirus, and doctor’s offices don’t test for it. Adriana Kajon wants that to change.

“Unless you look for it or you suspect it’s circulating or you are using diagnostic testing capabilities that can tell it apart, you are going to miss it, especially during flu season,” said Kajon, an infectious disease specialist at the Lovelace Respiratory Research Institute in Albuquerque.

“We are seeing severe adult infections,” Kajon told NBC News.

“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.”

Adenovirus infections often look like the common cold, or like influenza. They cause fever, headache, body aches and sometimes but not always cough, stomach distress and breathing problems. Some strains cause eye infections. There are 52 different strains.

Let our news meet your inbox. The news and stories that matters, delivered weekday mornings. This site is protected by recaptcha

Kajon and colleagues collected details of adenovirus infections from 2011 to 2015 at colleges, in nursing homes and elsewhere.

“On the basis of the severity of the clinical presentation of some cases in this study, the (adenovirus) vaccine currently licensed for military use should be considered a potentially valuable resource to prevent disease in susceptible populations living in closed communities, such as college settings, summer camps, and long-term care facilities,” they wrote in a report published in the journal Emerging Infectious Diseases.

An outbreak of adenovirus killed 10 people in 2007. Kajon’s team tested college students at one campus during the severe 2014-15 influenza epidemic and found 13 out of 168 students who came in for flu treatment had adenovirus infections.

Most patients may not suffer much, but the virus can cause very severe complications. Kajon and colleagues described the case of a 43-year-old women from Rochester, New York, who was previously healthy but became infected in 2012 and quickly developed pneumonia and respiratory failure. During her hospital stay she suffered brain swelling and bleeding and stayed on a ventilator for more than a month.

A year later, she was still out of breath if she exerted herself.

There was also the case of a 26-year-old Connecticut man infected in 2011 who had nausea, vomiting and chills. He spent days in the hospital with adenovirus infection.

There have also been outbreaks in long-term care facilities. But most clinics don’t test for adenovirus unless people are hospitalized with severe illness that isn’t helped by immediate treatment, Kajon said.

“These reports are probably the tip of the iceberg. We need more surveillance,” she said.

The Centers for Disease Control and Prevention keeps a log of reported cases of adenovirus.

“Outbreaks are more common in late winter, spring, and early summer but can occur throughout the year,” the CDC said.

There were so many outbreaks among new military recruits that the Department of Defense vaccinates personnel against two of the more serious strains with an oral vaccine. Vaccination was stopped for a few years in 1999 and outbreaks soared again, but there haven’t been many since the vaccine was re-introduced in 2011, the CDC said.

Kajon believes it should be more widely available,

“This is a vaccine-preventable disease,” she said. “A life is a life. Losing a loved one to viral pneumonia when you know it could have been prevented is hard.”

In the U.S., however, that requires catching the interest of a company that might want to develop such a vaccine commercially. "Unfortunately, it all comes down to the perception of having a market," Kajon said.

Adenovirus is not the killer that influenza is. Influenza kills 12,000 to 50,000 people a year in the United States alone, and puts up to 700,000 in the hospital. A cocktail of other viruses, from coronaviruses to rhinoviruses, also cause seasonal misery.

But Kajon said her studies show adenoviruse can be a significant part of the mix.

As with many viruses, there’s not a good treatment for adenovirus, although the antiviral cidofovir has helped some people with severe infections.

And adenoviruses are very hard to kill. Reports indicate they can survive on plastic and metal surfaces— think countertops and hospital tables— for a month. Some formulations of alcohol and chlorhexidine do not kill them easily, tests have shown, although chlorine does.