Chronic Lyme disease has hit celebrities such as singer Shania Twain. The ailment can cause muscle aches, mental fog, and fatigue for years or even decades.

With a number of celebrities and pro athletes revealing in recent months how Lyme disease has altered the course of their careers, awareness of the long-term effects of the disease is growing.

Researchers are still trying to figure out exactly how and why Lyme disease affects people in different ways and how best to treat them.

But evidence that the bacteria-borne disease sometimes sticks around in people’s bodies long after they’ve gone through initial antibiotic treatments appears to be mounting.

Singer Shania Twain said earlier this year that a struggle with dysphonia, which left her temporarily unable to sing, was caused by Lyme disease.

Pop star Avril Lavigne was bedridden for five months after contracting Lyme disease.

Pro golfer Jimmy Walker revealed in April that he was battling Lyme disease, and took a month off the PGA Tour as he recovered.

“I felt awful, like I had the flu every other week,” Walker, who thinks he contracted the disease in the fall, told The New York Times.

Months later, he is still taking 35 pills a day to treat his condition.

Read more: You have 48 hours to avoid Lyme disease after a tick bite »

Even when treated immediately, the effects of a bite from a Lyme-carrying tick may last — or unexpectedly reappear — long after the trademark bull’s-eye rash fades.

But the longer-term effects — and what to call them — are still a bit unclear.

“A lot of time people are talking of different things when they say ‘chronic Lyme disease,’” Dr. Adriana Marques, who leads clinical research of Lyme disease at the National Institutes of Health (NIH), told Healthline.

Chronic Lyme disease, late-stage Lyme disease, and post-treatment Lyme disease syndrome are all names for vaguely defined conditions in which people experience symptoms they and their physicians have trouble explaining.

Symptoms can include debilitating fatigue, muscle and joint pain, headaches, mental fog causing difficulty with memory or finding words, irritability, and sleeplessness.

Some people who experience these symptoms have been previously diagnosed with, and treated for, Lyme disease.

But others deal with symptoms similar to those experienced with Lyme disease, without ever getting a positive diagnosis for the condition.

The big question when it comes to chronic Lyme, Marques said, is what — since there is no evidence of Borrelia burgdorferi, the bacterium that causes Lyme, in some patients — is causing the symptoms.

Read more: Lyme disease is more common and more dangerous than most think »

The other big question is how to treat something with a cause that can’t be identified.

Antibiotics are sometimes prescribed on the theory that B. burgdorferi might still be hiding out in the body somewhere. However, although there have been anecdotal short-term successes, Marques said no studies have shown sustained benefits from antibiotics to people with chronic Lyme disease or those with post-Lyme disease syndrome.

The majority of people who contract Lyme and are treated for it with a course of antibiotics do get better with time.

But those who don’t — 10 to 20 percent, according to Marques’ review of the research — fall into the post-Lyme disease syndrome category.

Those people continue to experience persistent or intermittent symptoms a year after completing the antibiotics therapy.

Children appear less likely to develop long-term symptoms as are those who don’t delay antibiotics or have less severe cases of Lyme in the first place.

In cases of chronic Lyme, people who test negative for the disease despite symptoms could be infected with another tick-borne illness or have an autoimmune disorder or other problem.

How to treat these long-term symptoms is still a mystery.

For now, Marques and her team are working on identifying biomarkers and other ways to find out definitively whether Lyme disease is to blame for the symptoms.

That additional research is becoming more critical.

Researchers have predicted higher tick numbers in some parts of the country this summer.

Tick ranges have been expanding due to warmer winters, and more people are getting bitten by ticks as cities continue to sprawl out into wooded areas.