Sept. 4, 2018 -- In May 2015, Jackie Shelton, a public relations professional and mother of two in Reno, NV, noticed a rash on the back of her leg. She'd been working in her yard and thought nothing of it. Then she awoke in the middle of the night in excruciating pain.

"The pain was emanating from inside my leg and radiating up to the surface," she says. "It was like fire ants inside my body, on my nerves."

A trip to urgent care led to an unexpected diagnosis: She had shingles.

Shelton was 50 years old. At the time, she was considered 10 years too young to get the Zostavax shingles vaccine, which was recommended for people ages 60 and older.

"I had really not paid any attention to shingles until I found out I had it. Everything I read said it affected older people, so that was a surprise," she says.

Katie Ochoa was just 28 when she got the telltale rash. "I called the doctor's office and said, 'I think I have shingles.' They laughed and said, 'You don't have shingles.' As soon as I pulled up my shirt they were like, 'Wow, this is shingles.' "

Shingles is known as a disease that mainly strikes older adults, because the varicella zoster virus that causes it often emerges from its slumber late in life. Varicella is the same virus responsible for chickenpox. After you recover from chickenpox in childhood, the virus lies dormant in your nerve cells. It can reawaken as the painful, blistering shingles rash when your immune system naturally weakens with age.

Over the last 6 decades, there has been a steady uptick in the number of shingles cases in the United States -- even among younger adults. A 2016 study found that rates of shingles have been climbing since the mid-1940s in all age groups. From 1945 to 1949, 0.76 out of every 1,000 people got the disease. Between 2000 and 2007, that number rose to 3.15 people per 1,000.

The virus has hit older adults particularly hard. Shingles rates rose 39% from 1992 to 2010 in people over 65.

What's behind the increase in shingles cases? Researchers have a few theories.