One of the more romantic Christmas traditions is kissing under a sprig of mistletoe, hung from a ceiling or doorway. Those who find themselves under its green leaves might not know that this symbol of love is actually a vicious parasite that survives by sucking the nutrients from trees.

Don’t think that making out under a parasite sounds very romantic? Well, it gets weirder. The plant’s parasitic nature is probably why people began to think mistletoe was special enough to kiss under in the first place.

Mistletoe seems to miraculously stay green all winter, and this is "the fundamental basis of all mid-winter traditions relating to mistletoe,” says Jonathan Briggs, a mistletoe expert and consultant. But it keeps that lively green color by stealing water and soil minerals from its host tree.

Though some mistletoe species “can be fairly benign in limited numbers,” others take so much from the tree that they “can be problems in any quantity,” says Briggs. If either type spreads to most of a tree’s branches, the tree will die.

“And so,” he says, “will the mistletoe.”

Throw in the fact that some species are poisonous, and mistletoe starts to seem less like something you’d spy mama kissing Santa under and more like something Krampus would plant on your Christmas tree.

But that doesn’t mean that mistletoe is all bad. Scientific evidence suggests that some species of mistletoe could have serious benefits for birds—and humans.

The Benevolent Parasite

Even though mistletoe kills trees, studies have shown that when some species are removed from their ecosystem, birds suffer. According to a 2012 paper in the Proceedings of the Royal Society B, when scientists cleared mistletoe from parts of Billabong Creek in New South Wales, Australia, they saw a significant drop in the population of birds and other species. In the areas with mistletoe, those populations stayed the same or increased.

A 2014 study published in Acta Oecologica made similar observations about the relationship between mistletoe and birds in central Mexico. Both papers argue that mistletoe is a keystone species—an organism that plays a crucial role in its ecosystem.

There’s also evidence to suggest that some species of mistletoe can be used in treating cancer—something that people have actually used it for since the 1920s. Doctors today can prescribe mistletoe in Europe. And at Johns Hopkins’ School of Medicine, doctors are performing the first rigorous, I.V. study of mistletoe’s effects on cancer patients in the U.S.

According to oncologist Channing Judith Paller, previous research suggests that the parasite could at minimum help “ameliorate side effects of traditional therapy,” such as the nausea, vomiting, and loss of appetite that often accompanies chemotherapy.

“If those symptoms go down, the patient can then tolerate the chemotherapy better and take more of it,” says Paller.

Maybe mistletoe is nice enough to kiss under after all.

The Sensual Parasite?

It’s not clear why people began kissing under mistletoe. One Norse myth claims that, after the god Balder was killed with a mistletoe arrow, people began to kiss under the parasite. The details of why they started kissing under it are somewhat fuzzy—it might have something to do with making peace under the plant.

Other origin stories say that people started kissing under mistletoe because it was a sign of fertility; and there are some physical clues as to why people may have thought this. Besides the fact that many species stay green in winter, some species have large berries that secrete what some have described as a semenlike substance.

In the U.S., kissing under the mistletoe used to be a lot more complicated. Washington Irving wrote that men commonly gave women as many kisses as there were berries on the mistletoe hanging above them, plucking off one per kiss. Hopefully, these couples never performed the ritual with the large white berries of dwarf mistletoe—which, in a move of evolutionary genius, spread their seeds by exploding.

A symbol of fertility indeed.