Most people probably know by now that flu can make fall and winter dangerous for older adults.

But another virus that circulates at the same time and causes similar symptoms is now getting more attention as a killer of older people, especially those who are frail, or have chronic health problems or weakened immune systems.

“There’s another flu out there,” said William Schaffner, medical director for the National Foundation for Infectious Diseases (NFID) and professor of preventive medicine and infectious diseases at Vanderbilt University School of Medicine.

Long thought to be a threat mostly to infants, respiratory syncytial virus (RSV) is increasingly recognized for its impact on adults.

The NFID released a report on RSV last year that called it a "hidden epidemic" and estimated that it causes 14,000 deaths and 177,000 hospitalizations a year of people 65 and older in the U.S. As a cause of severe respiratory illness, it is second only to flu, which the Centers for Disease Control and Prevention says leads to at least 12,000 deaths a year.

In kids under 5, the CDC estimates that RSV causes 57,000 hospitalizations a year. A recent study found that deaths due to the virus are rarer in the U.S. than once thought, but another analysis found that, globally, the virus causes 60,000 pediatric deaths a year and 3.2 million hospitalizations.

Schaffner is part of a CDC consortium trying to document RSV’s impact among the elderly; hospitalization and death data in this population now are out-of-date and squishy.

Most people contract the virus by age 2 and get it repeatedly throughout life.

For healthy people, RSV feels like a cold. Its symptoms can be indistinguishable from the flu, except for the fact that RSV is less likely than the flu to cause fever.

Unlike the flu, there's no vaccine for RSV, but infectious disease specialists hope that will soon change.

In its report, the NFID said more than three dozen RSV vaccines are in development, and a dozen are in clinical trials. Some target older adults and pregnant women, with the goal of protecting their babies. The efforts are haunted by a pediatric vaccine trial in 1967 that actually made the disease worse.

Novavax, a Gaithersburg, Md., company, is among those racing to develop an RSV vaccine. It had a setback last year — its vaccine failed in a Phase III clinical trial — but the company is now testing a different approach.

Lou Fries, senior vice president and chief medical officer for the company, said RSV is a challenging virus that "appears to be able to modulate our immune responses to it" in ways that researchers don't fully understand. Unlike influenza virus, RSV doesn't change much from year to year, but even after contracting it, our bodies don't mount long-lasting immunity to it.

Novavax is now testing whether it can boost the impact of the vaccine by adding an "adjuvant" it calls Matrix-M that could help compensate for the fact that as people age, their immune systems naturally weaken. The goal, Fries said, is to produce a more "efficient" immune response and higher-quality antibodies to combat RSV infection.

There's no specific treatment for RSV in adults, and you prevent it the way you prevent other airborne viruses: with frequent hand washing, covered coughs and sneezes, and avoiding sick people. People can get re-infected after a year or so. In hospitals, all people with flu-like symptoms are supposed to be isolated to protect other patients.

Why should we care about RSV if there's little we can do to stop or treat it?

"More often, we make better decisions if we know what we're doing," Schaffner said. There's no point in giving Tamiflu, a treatment for the flu, to patients with RSV. Knowing what's causing symptoms could reduce overuse of antibiotics. Doctors also want an understanding of who is most vulnerable to figure out who should get vaccines.

Generally, knowing more about RSV could help patients understand why they're sick even though they got a flu shot — and that could improve use of the vaccine.

Improved testing is making it easier for doctors to know which virus people have. In the past, testing was time-consuming, and doctors had to guess which test to order, said Thomas Fekete, chief of infectious diseases at Temple University Hospital. Now they can use kits that quickly test for a panel of eight or more flu-like illnesses. The kits cost $200, though, and some hospitals use them sparingly.

Judith O'Donnell, director of infectious diseases at Penn Presbyterian Medical Center, said her hospital now uses the testing panels routinely. Flu causes most of the severe disease she sees, she said.

Fekete said Temple tests sicker patients and those who are at higher risk for problems, such as transplant patients. The results have been surprising, he said.

At the busiest time of the year — January and February — Fekete estimated that 30 percent to 40 percent of the people sick enough to get tested had RSV. Their average age is older than that of the flu patients, who account for another 40 percent of the test results, he said.

“There’s a lot of people that have viruses other than flu,” said Fekete.