"There's no known source of caffeine in temperate zones, so there's nothing we can suggest as a sort of alternative hypothesis at this point except for a human source, " she said.

CNN: Researchers find caffeine in waters off Oregon

Writing about his work earlier this year in Virology Journal, Rice shows how chickenpox is much less common in parts of the world with high levels of UV rays compared with places where the levels are low, and why in temperate zones, the disease peaks in winter and spring, when UV rays are at their lowest.

FORBES: Sun's UV Rays May Prevent Chickenpox