For most of the time that humans have walked on Earth and scratched at itchy, red rashes, there was no reason to think chicken pox and shingles are related. They look so different.

Chicken pox usually strikes small children. It manifests as red bumps, eventually distributed over the whole body. In the 18th century, a German doctor dubbed chicken pox “varicella,” a diminutive of his name for smallpox or “variola,” because chicken pox seemed to be its less severe form. Shingles, on the other hand, usually affects adults, and its rashes often appear as an angry red stripe across one side of the torso. This characteristic shape led ancient Greeks to call it “herpes zoster,” or roughly “creeping belt.”

It was not until the the invention of powerful microscopes in the 20th century that the two became linked. Scientists could finally see that the diseases were in fact caused by the same virus, the now awkwardly named “varicella zoster virus.”

The virus, it turns out, never quite leaves the human body. It lurks inside neurons even after chicken-pox symptoms have cleared. Decades later, it can reawaken as shingles and infect skin cells along a single nerve, giving rise to the distinctive red stripe. The reactivated virus can also find new human hosts, born after the last outbreak and vulnerable to chicken pox. It’s a clever evolutionary gambit—you have to give it that.

On Friday, the Food and Drug Administration approved a new vaccine from GlaxoSmithKline to prevent shingles in adults age 50 and older, but not chicken pox. The two diseases may arise from the same virus, but they afflict different populations and that is key: The immune systems of the elderly susceptible to shingles are different from the immune systems of children susceptible to chicken pox.

GlaxoSmithKline’s shingles vaccine contains a lone protein isolated from the shell of the varicella zoster virus. This protein acts as an ID tag, allowing the immune system to recognize all future varicella zoster viruses it encounters.

On the other hand, the existing chicken-pox vaccine, aimed at children, contains a whole, live, but weakened varicella zoster virus. “Live viral vaccines tend to work better in children,” says Ann Arvin, an infectious-disease specialist at Stanford University. (Arvin has consulted for both GlaxoSmithKline and Merck, which makes the chicken-pox vaccine.) The reasons are not entirely well understood, but children have a different mix of immune cells than adults, and live viruses tend to provide better, broader immunity through life.

But live viruses, even weakened in a vaccine, can pose problems for people whose immune systems are weak due to age or disease. Such vaccines could make them sick. There is an existing shingles vaccine, made by Merck—basically a super-large dose of its live chicken-pox vaccine—but it is not recommended for immunocompromised patients.

It also works less well than GlaxoSmithKline’s new vaccine, which is more than 90 percent effective. “It’s a real paradigm shift because there are no vaccines that perform so extraordinarily well for people in their 70s and their 80s,” says Rafael Harpaz, an infectious-disease specialist at the Centers for Disease Control and Prevention. Scientists attribute its stunning effectiveness to a new adjuvant, an additional chemical in the vaccine that primes the immune system for the viral protein. A CDC panel on Wednesday actually voted to recommend GlaxoSmithKline’s new shingles vaccine even for people who had received Merck’s old shingles vaccine.

But could GlaxoSmithKline’s shingles vaccine work to prevent chicken pox as well? “We don’t know that, and I’m not sure if we ever will,” says Anne Gershon, a pediatric-disease specialist at Columbia University. (Gershon has received research funding from Merck and consulted for GlaxoSmithKline.) We might never know because someone would have to test it—and given that a safe, effective chicken-pox vaccine already exists, it’s unlikely anyone will ever take the risk. A GlaxoSmithKline spokesperson confirmed the company has no plans to test its vaccine for chicken pox.

Conversely, the chicken-pox vaccine does seem to offer some protection against later occurrences of shingles. The weakened varicella zoster virus strain in vaccines also lurks dormant in neurons, but it does not reawaken so easily. Kids who got chicken-pox vaccines are less likely to later get shingles than kids who naturally caught chicken pox.

Why shingles flares up at all is still largely a mystery. True, it often happens when a patient’s immune system is weak, like if they are older or have taken immune-suppressing drugs. But why does it also sometimes happen in seemingly healthy people? “We are clueless, and we’re on the chase,” says Harpaz.

Harpaz has questioned the conventional wisdom that stress can lead to shingles. A few years ago, he got access to a large insurance data set, in which he could identify people whose spouses had became catastrophically ill or died unexpectedly. These people, presumably, would be under a lot of psychological stress. While the surviving spouses did in fact seek mental-health treatment in higher numbers, they did not seek more treatment for shingles. He and his colleagues wrote up a paper titled, “Psychological Stress as a Trigger for Herpes Zoster: Might the Conventional Wisdom Be Wrong?” Like most articles with questions in their titles, it doesn’t provide a definitive answer. Harpaz is still on the case.