The Brood - the X-Men's version of the Xenomorph from Aliens - are more dangerous than anyone realizes, simply because their powers are mystical in nature. Marvel's best comics have always riffed on popular culture, and the alien race known as the Brood are a particularly obvious example. The Brood implant themselves within a host, in a manner so very similar to the Xenomorphs. The key difference, though, is that the host actually becomes a Brood, meaning the aliens gain access to all their powers.
The Brood are a scourge upon the galaxy, one that has consumed entire races and civilizations. The X-Men's version of the Xenomorphs have threatened Earth on many different occasions, but fortunately they've been driven back by Earth's defenders. They've always shown particular interest in the potential of mutant hosts, simply because the Brood can gain mutant powers with particular ease. They once even took possession of Professor X and manipulated him into gathering together a team of new mutants the Brood could prey upon. And yet, as often as they may have focused on mutants, there's some evidence the Brood are actually mystical in nature.
The Brood Seem To Be A Race Of Alien Demons
The most important piece of evidence lies in the fact that Brood can possess supernatural entities as well as mutants. The Brood demonstrated this power in a classic X-Men/Ghost Rider crossover from 1992, "Brood Trouble in Big Easy." Depicted in X-Men #8-9 by Jim Lee and Scott Lobdell and Ghost Rider #26-27 by Howard Mackie and Ron Wagner, this story sees the Brood actually possess Ghost Rider, absorbing all his demonic power into their hive. The Ghost Riders' powers are not genetic in nature, so this should be impossible. It's a massive hint that the Brood don't quite work the way most readers believe they do; they must use some manner of dark magic to possess a host.
This theory is supported in New Mutants #63 by Chris Claremont, Louise Simonson and Bo Hampton, a strange issue in which Magik encounters the Brood. She is able to take a group of Brood-infected mutant clones to the Hell Dimension of Limbo, where she is surprised to discover she is able to perceive the Brood eggs. Even more striking, she can kill these eggs using her Soulsword - a weapon that, at the time, was only able to affect things that are magical in nature. The issue presents the story as though it could be just a dream, but it all seems so real, and Magik is left confused at what she believes she has experienced. The implication is that Magik's strange encounter with the Brood was very, very real - and that she unwittingly discovered their true nature.
The Brood are returning to attack the X-Men again, so it will be interesting to see whether this idea is developed at all. Ironically, it means the Brood would have been wiser not to focus on mutants. They should instead have focused their attention on Doctor Strange and other sorcerers, because they can presumably absorb all the mystic power of these beings - as they did with Ghost Rider. The Marvel Universe may have reason to be glad the Brood tend to be neutralized by the X-Men instead.