Last week Feinstein, 89, said in a statement that she had suffered complications from the condition, which occurs when the varicella-zoster virus—responsible for chickenpox—reactivates later in life. 

The Democratic stalwart experienced encephalitis, or inflammation of the brain, which “resolved shortly after she was released from the hospital in March,” a spokesman for the senator told The Associated Press.

And she continues to battle Ramsay Hunt syndrome, a rare neurological disorder seen in some people who have, or have recently had, shingles. The syndrome occurs when the reactivated varicella-zoster virus spreads to a facial nerve, according to the National Organization for Rare Disorders. While the syndrome, like shingles, is typically only seen in seniors, pop star Justin Bieber, 29, announced last year that he was suffering from the condition. His condition has since improved.

When Feinstein returned to the Senate on May 10 after more than two months away, her face appeared partially paralyzed, leading to speculation that she had suffered a stroke. She is still recovering from Ramsay Hunt syndrome and will operate on a reduced schedule, aides told the AP last week.

The senator’s illness has put shingles back in the spotlight—and attention to the matter is much needed, experts say. A staggering 99% of people born before 1980 have experienced chickenpox—and anyone who has is at risk for developing shingles later in life.

Here are seven things you need to know about the exceedingly painful condition, known for striking without much warning—even among those who think they never had chickenpox.

Shingles occurs as your immune system weakens, usually with age.

Once someone is infected with the varicella-zoster virus, a type of herpes virus that isn’t sexually transmitted, it retreats to the nervous system, where it remains perpetually. It can reactivate as the immune system weakens—often due to aging, and typically after age 50, Dr. Sajida Chaudry, a primary care physician and the Office Medical Director at Johns Hopkins Community Physicians at Odenton, recently told Fortune’s Alex Vance.

But it can occur at age, at any point, in anyone who’s had chickenpox.

Those who are immunocompromised—including those undergoing chemotherapy and other cancer treatments, and those who have HIV—are among those at higher risk, Chaudry said.

“I’ve seen very healthy people get shingles, and sometimes there isn’t any [specific] reason,” she added.

Don’t think you had chickenpox? Think again.

If you don’t remember experiencing chickenpox as a child, don’t assume you’re

in the clear. It’s possible to have a mild course of the virus with no symptoms,

according to Chaudry.

One early sign of shingles: a strange tingling sensation.

Shingles occurs as the dormant virus reactivates from within your body’s nerves. As this happens, you might feel a strange tingling sensation, Chaudry said.

“The skin just feels different,” she explained. “Some days later, [you may] notice a rash. It’s usually red spots that slowly become [similar to] blisters, usually a whole crop of them. They can range from mildly uncomfortable to very painful.” 

The main symptom of shingles is a rash—but more rare complications can occur, like in Feinstein’s case.

A shingles rash usually occurs in one area of the body, like the chest, back, or abdomen. Those who are older or immunocompromised may experience a more severe rash. The rash typically clears in a week or two, but can take longer to heal if it’s more severe. If you have shingles, your doctor will likely prescribe you an antiviral, and also recommend symptomatic treatment like over-the-counter pain medication, calamine lotion, and oatmeal baths.

Other common symptoms include fatigue, headache, fever, chills, and/or an upset stomach. More rare symptoms include ocular shingles, which can lead to loss of vision, eye inflammation, and severe pain; hearing problems; encephalitis; and pneumonia.

Those who have shingles can’t pass it on—but they can give someone chickenpox.

Contact with fluid from a shingles rash can transmit the varicella-zoster virus, potentially resulting in chickenpox—not shingles—in someone who has never been exposed. After experiencing chickenpox, the newly exposed person could later develop shingles.

If you have shingles, be aware that your rash is contagious until it scabs over. Keep it covered, avoid scratching or touching it, and wash your hands frequently, Chaudry advises.

There’s a shingles vaccine—and everyone age 50 and older should get it.

As with all vaccines, the shingles jab isn’t a guarantee that you won’t experience the blisteringly painful condition. But it reduces your chance of developing it, and it should make your symptoms more mild if you do, Chaudry says.

It also reduces your risk of developing post-herpetic neuralgia, or chronic shingles pain, which occurs in about 10% to 20% of those who’ve had shingles. That condition can cause pain for months or even years after the skin has healed, she advises.

Chaudry recommends that anyone aged 50 and older make getting the two-dose vaccine a top priority—even if they’ve already had singles, as it’s possible to get it multiple times.

“Prevention is always better than cure, and we have the luxury of timing these things,” she said. “When you go to the doctor for your annual, that is the best time. It really is the best thing for your health and [for] taking care of yourself.”