People who develop shingles -- a re-activation of the chicken pox virus which commonly affects older people -- may face a ballooning risk of heart attack or stroke, South Korean researchers said Monday.

Their study found that people with shingles saw an overall 41 percent higher risk of cardiovascular events, such as heart attack or stroke, when compared to an age-matched control group that did not develop shingles.

The risk of stroke was 35 percent higher and heart attack 59 percent higher, said the report published in the Journal of the American College of Cardiology.

The riskiest period was the first year after infection, and the dangers appeared to decline after that.

Researchers also found the risk for stroke was highest in those under 40 years old.

The study was based on a medical database of 519,880 patients whose records were tracked from 2003-2013.

Researchers are unclear why shingles would boost the risk of cardiovascular problems, and said more study is needed.

"While these findings require further study into the mechanism that causes shingles patients to have an increased risk of heart attack and stroke, it is important that physicians treating these patients make them aware of their increased risk," said study author Sung-Han Kim, a physician in the department of infectious diseases at Asan Medical Center in Seoul.

Nearly one in three people in the United States will develop shingles in their lifetime, according to the US Centers for Disease Control and Prevention.

The infection, which may cause blisters, rash and shooting pain, can affect anyone who has had chicken pox.

Shingles is a disease caused by the varicella-zoster virus. It can be spread through direct contact with the rash, but not by air.

A vaccine against shingles is available and is recommended for people 60 and older, according to the US National Institute of Allergy and Infectious Diseases.