Sen. Feinstein’s high-stakes battle with shingles shows just how serious the disease can be. Learn how to spot the symptoms and stay safe
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California’s U.S. Sen. Dianne Feinstein fought a more harrowing battle with shingles than was previously disclosed, her office said Thursday, as it revealed that the condition lead to complications that partially paralyzed her face and caused her brain to swell.

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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.