What's the difference between long COVID, long cold and long flu?

COVID-19 has brought intensified focus on infectious diseases, including scientists learning more about common respiratory illnesses in the wake of the pandemic. That has led to the discovery of so-called "long" respiratory illnesses, which cause lingering symptoms well after the initial infection clears up.

While many people are aware of long COVID (aka post-COVID conditions), recent research has discovered that long flu and long cold are also a thing. But what are long cold, long flu and long COVID, and when should you see a doctor if you suspect that you have one of these? Infectious disease experts break it down.

What turns a respiratory illness into the long version?

While the idea of long COVID has been established for years, research only identified the concept of long flu and long cold over the past few months. But in general, doctors agree that respiratory symptoms that last more than three months puts you into the long category.

"We've settled on the three-month marker," Dr. William Schaffner, an infectious disease specialist and professor at the Vanderbilt University School of Medicine, tells Yahoo Life. "If you have symptoms beyond three months, we're calling that 'long COVID' and that seems to be the consensus for other long respiratory illnesses as well."

Were long versions of the flu and cold always around?

Dr. Amesh Adalja, a senior scholar at the Johns Hopkins Center for Health Security, tells Yahoo Life that long respiratory illnesses were not much noticed before COVID-19, but it seems that they were already around. "We already knew that symptoms of any sort of respiratory infection can last more than a week, and that certain viruses can do damage to your respiratory tract that can leave it irritated for a while," Dr. Thomas Russo, chief of infectious disease at the University at Buffalo in New York, tells Yahoo Life. "We're paying more attention to long illnesses now with COVID and our awareness of long COVID."

Schaffner agrees. "Retrospectively, we have seen that other viruses have caused something similar to long COVID, although perhaps not as pronounced," he says. "COVID has helped us to define these and bring them more to attention."

Research has found that long cold and long flu can cause symptoms such as a lingering cough, trouble sleeping, memory issues and trouble with taste and smell, although these symptoms are usually more severe with long COVID.

When to see a doctor about a long respiratory illness

Doctors say it's important to be aware that you can develop a secondary sickness from a respiratory illness, such as bronchitis or pneumonia — and that's different from a long illness. "The tell-tale signs of a bacterial infection on top of influenza, COVID or a cold is often that a person might start to feel better and suddenly develop a fever, increased shortness of breath and chest pain," Russo says. Schaffner agrees, adding: "That should signal to you that you're getting something more complicated, and you should call your doctor."

But long cold, long flu and long COVID are different. These tend to have more vague symptoms and don't usually come with a fever, Adalja says. Symptoms of long infections can also make it hard to go about your day as you normally would, he says. "However, it is important to evaluate longer-term symptoms to rule out the possibility of a secondary infection," Adalja says.

If you had a cold, the flu or COVID-19 and are still dealing with symptoms three months later, Russo says it's time to call a doctor.

How are long respiratory illnesses treated?

There is unfortunately no set treatment plan for long cold, long flu or long COVID. "It's really treating symptoms at this point," Russo says. "We're still sorting out what's causing it — that makes treatment difficult."

If you suspect you have a long respiratory illness, Russo recommends contacting your health care provider for an evaluation. And, if you think you have long COVID, Schaffner suggests trying to find a major medical center near you that has a long COVID clinic. "A number of medical specialities come together to treat patients in these," he says.

Overall, Russo says that the odds of having long cold, long flu or long COVID seems to be related to how serious the infection is. There are a lot scientists, however, that still don't know about these long illnesses. "Hopefully, research will give us more answers," he says.