Warning: The following contains spoilers for Avengers #27 by Jason Aaron, Ed McGuinness, Mark Morales, Jason Keith and Cory Petit, on sale now.
There are numerous invasive alien species in the Marvel Universe. Some of them use force to meet their goals, such as the Kree. Others, like the Skrulls, utilize stealth and infiltration to achieve their ends. But almost none are as frightening as the Brood, a species of insectoid monsters that can turn other species into members of their own genus as a means to overwhelm and swarm the entire universe.
Now, the Brood has been able to infect Thor and turn him into one of their own -- which could be a big problem for the rest of the universe.
THE PROBLEM WITH BROOD
Essentially, the Marvel Universe version of the Xenomorphs from the Alien franchise, the Brood are one of the most fearsome and threatening species in the universe. They are an intergalactic race of invasive aliens that spread their number across the galaxy by infecting other organisms with their eggs. Upon hatching inside the other beings, the Brood forcibly change their hosts into a Brood version of themselves, taking their powers for their own.
This process also overwrites the mind of the infected, turning the new hybrid creature into a committed and blood-thirsty member of the Brood Hive. With only a handful of exceptions (such as Broo from Wolverine and the X-Men) the Brood are dedicated to spreading themselves across all of creation until the only thing alive in the universe is the Brood Hive.
THE BROOD THOR
In Avengers #27, The Avengers team (sans Black Panther but with Black Widow) have traveled into space to learn more about the mysterious appearance of a new Starbrand in Shi’Ar space. The mission doesn’t go as well as planned, with the team quickly separated and without any real way to reach out for help. While some have found themselves in the back of Ghost Rider’s hell-charger, others have been attacked and remain in the ship they used to take to the stars in the first place. Notably, Captain America and Thor remained behind while their remaining teammates tried to find a way to survive.
Captain America has been creating a captain's log for prosperity. Meanwhile, Thor has essentially cordoned himself off from his longtime ally because he has somehow been infected with a Brood egg and begun the physiological transformation that comes with it. As Thor tries to fight off the infection, his connection with the hammer Mjolnir becomes more and more important. If he can still lift it, it means there's still enough of him left to make him worthy. The moment he can no longer carry the hammer, though, the infection has completely transformed him into a Brood.
Although it’s possible to purge someone of a Brood infection, it can be very difficult. What’s surprising, however, is that there’s apparently a strain of Brood that have the ability to infect a powerful deity like Thor. Other beings with advanced healing abilities -- such as Wolverine -- have proven to be too tough for the Brood infection to spread. This suggests that for one to successfully have infected someone as powerful as Thor, something must be amiss.
It’s even possible that Thor has been infected by a Brood Queen egg, which could explain how it was able to overcome Thor’s typical immunity to most infections or sickness. If that’s true, then Brood-Thor might be one of the single most dangerous beings in the universe.
He could continue to spread the Brood infection, creating more and more members of the Hive around the world. He could even create new Brood, allowing the infection to spread to other groups like the Avengers or the Asgardians. An army of Brood-Gods could potentially wipe out most life in the galaxy. Now, the only thing standing in the way of that potential future is Captain America -- alone and without almost any backup.