If you’ve ever had chickenpox—and a staggering 99% of people born before 1980 have—you’re at risk for developing shingles later in life. But why is that, exactly?

The answer lies in the varicella-zoster virus (VZV), which causes chickenpox. Once you recover from chickenpox, the virus stays in your system permanently and can reactivate as shingles—a painful, blister-like rash—years later. Although chickenpox and shingles are caused by the same virus, they are not the same illness.

What causes the chickenpox virus to reactivate?

“[The chickenpox virus] is always [in your body], it’s just dormant. It stays in the nervous system. As we get older, it can get reactivated,” says Sajida Chaudry, MD, MPH, MBA, a primary care physician and the Office Medical Director at Johns Hopkins Community Physicians at Odenton.

The reactivated herpes zoster virus is also known as shingles.

The main reason it reappears? Your immune system tends to weaken with age, causing the once-dormant chickenpox virus to reawaken, typically after age 50. Dr. Chaudry, who is board certified in preventive medicine and family medicine, adds that people who are immunocompromised, including those undergoing chemotherapy and other cancer treatments, or those with HIV, are more susceptible to a shingles outbreak.

That said, anyone who has been infected with chickenpox at any point is at risk. “I’ve seen very healthy people get shingles, and sometimes there isn’t any [specific] reason,” Dr. Chaudry tells Fortune.

And here’s the kicker: You may have had chickenpox without ever knowing it, putting you at risk for shingles even if you think you’re in the clear. “That’s the biggest misconception,” emphasizes Dr. Chaudry. “[Chickenpox] may have been mild enough that you didn’t have symptoms, but it doesn’t mean you weren’t exposed.”

While having chickenpox does not automatically lead to shingles, the CDC estimates that 1 in 3 adults will develop the rash, affecting around 1 million people per year.

Spotting the early signs of shingles

Because the dormant chickenpox virus lives in your body’s nerve cells, one of the initial signs it’s reactivating is a strange, tingling sensation around the affected nerves.

“The skin just feels different,” explains Dr. Chaudry. “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.”

She points out that the rash is typically concentrated in one area of the body—most commonly on the chest, back, or abdomen. Those who are older or immunosuppressed may experience a more severe rash.

Other symptoms can sometimes include fatigue, headache, fever, chills, or upset stomach. It is best to contact your healthcare provider as soon as initial symptoms appear to ensure timely treatment.

In 10% to 20% of cases, a shingles rash can affect the nose, forehead, or eyelids and lead to eye damage—also known as herpes zoster ophthalmicus, or ocular shingles. Complications include loss of vision, eye inflammation, or severe pain.

“It is definitely more worrisome if [shingles] is around the face because we have to make sure it’s not impacting the eyes,” adds Dr. Chaudry. Other rare complications of shingles can include hearing problems, brain inflammation, and pneumonia.

When it comes to shingles treatment, the first step is to tackle the virus with an antiviral prescribed by your doctor, says Dr. Chaudry. The next step is managing the pain, usually with over-the-counter pain medication or other measures such as wet compresses, calamine lotion, and oatmeal baths.

If the pain is severe, a prescription from your doctor may be necessary. The rash typically clears within 1 to 2 weeks, adds Dr. Chaudry, although it may take longer depending on its severity.

Is the shingles rash contagious?

Here’s where things get a bit complicated: You cannot get a shingles rash from someone who has it—but you can get chickenpox if you’ve never had it before.

If you’ve never been infected with varicella-zoster (or never received the chickenpox vaccine), contact with fluid from a shingles rash can spread the virus, which may result in chickenpox. The virus then stays in your system and can reactivate later on, causing shingles.

To prevent varicella-zoster transmission, it is crucial to keep a shingles rash covered, avoid scratching or touching the rash, and wash your hands frequently. The rash is still considered contagious until it scabs over.

Why the shingles vaccine is so important

According to Dr. Chaudry, the shingles vaccine doesn’t just help prevent a shingles outbreak (and the Shingrix vaccine is around 90% effective at doing so, according to the CDC), but it also lessens the severity and duration of symptoms if it does occur.

“[The vaccine] is trying to prevent you from getting [shingles], but if you do get it, it will be milder,” she says.

The vaccine also lowers your risk of developing postherpetic neuralgia, or chronic shingles pain. This occurs in 10% to 18% of shingles patients, causing long-term nerve pain in the months or years after the rash has cleared.

“The [main] risk factor [for postherpetic neuralgia] is age,” explains Dr. Chaudry, noting that chronic shingles pain is more likely to occur in adults over 60 years of age.

The two-dose shingles vaccine is available to anyone 50 and older and should be a top priority for your health, says Dr. Chaudry. “The immune system changes with time, so it’s best to protect yourself as soon as possible,” she adds.

As for those who have already had shingles? It’s a question Dr. Chaudry receives often, and her answer is always the same: “You should get the vaccine even if you’ve already had the shingles rash. Unfortunately, you can get it again. It’s not common, but it does happen.” Typically, recurrent shingles is more likely to occur in elderly or immunocompromised patients.

Because you may have been exposed to chickenpox without knowing it, the best way to avoid shingles is to get the vaccine as soon as you’re able to. It is important to speak with your doctor about any allergies you have prior to receiving the vaccine.

“Prevention is always better than cure, and we have the luxury of timing these things,” says Dr. Chaudry. “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.”