Shingles results from a flare-up of the varicella-zoster virus (VZV). When the virus is contracted, which normally happens during childhood, chickenpox occurs. Once chickenpox clears up, the virus becomes latent in some nerve cells in the body, commonly for years or decades.
Even if the virus is contracted during adulthood (which is rare), it initially causes chickenpox and only a recurrence will lead to shingles. It is also possible for the virus to flare-up again, leading to another shingles outbreak.
Although the shingles rash can lead to infection with VZV in people who have never had the chickenpox, people infected with VZV by someone with shingles will develop chickenpox, not shingles. It is important for people with shingles to avoid contact with those who have never had chickepox, especially pregnant women. This is because initial VZV infection during pregnancy can lead to serious birth defects.
The specific reasons an individual experiences a recurrence of VZV are not known. However, two associated risk factors are age (most cases occur in adults over the age of 50) and having a weakened immune system, according to the Mayo Clinic. The immune system can be weakened through disease, such as advanced HIV-disease or through medical treatments, including radiation and chemotherapy for cancer as well as steroids.
The shingles vaccine is available as a preventative measure against shingles. Although vaccination does not guarantee that an individual will not develop shingles, it does decreases the chances of both shingles and postherpetic neuralgia, a chronic painful condition that is a complication of shingles, according to the CDC. The vaccine is recommended for use in people over the age of 60 who have healthy immune systems. The vaccine can also be used in people who have already had an outbreak to reduce their chances of having another outbreak.
Some people who are vaccinated against shingles develop a shingles-like rash. According to the CDC, no cases have been reported of someone transmitting VZV to another person through this vaccine rash; however, the rash should be covered as a precautionary measure.