Claremont has said they were inspired by the xenomorphs of the 1979 movie Alien but, interestingly, the Brood appeared before the xenomorph’s hive society - and their Brood-resembling Queen - were introduced in the sequel Aliens.
The Brood have the ability to impregnate a host (of many lifeforms) with an egg. When the egg hatches, the host is "morphed" into a Brood, and is effectively dead. The newborn Brood has any abilities the host had, and can "morph" back into the host in order to appear like the host's race. Finally, a Brood can "morph" into a hybrid form, with physical characteristics of the host lifeform and the Brood.
There are several types of Brood: First of all, the Broodlings, which are brown, smaller and winged (and were nicknamed "sleazoids" by Wolverine) and their larger Brood Queens, also brown. The Broodlings have flexible abdomens that they need to coil most of the time, and have transparent wings. The larger Queens are wingless and have round abdomens. All Brood obey the mental call of an Empress Brood. The Empress is green in color, and extremely large. Her offspring, the Firstborn, are "pure" and have had no "mingling" with other races, and as such, are green and larger, as well.
Due to their natural body armor and teeth, the Brood are very dangerous in combat. In addition, they have stingers that can deliver either paralyzing or killing poison. Sleazoids can fly, while the Queens have the ability to implant Brood embryos in other beings. The Queens can also communicate with their spawn by telepathy, even across interstellar distances.
Some Brood have been shown as being able to switch back-and-forth between their host's form and their true one, even changing into a hybrid form if they wish (for example, humans with brood-like jaws and tentacle arms).
The Brood have a civilization based on the typical communal insect societies, such as those of the bees and ants. The Queens are the absolute rulers, while the sleazoids do all the work; despite their evil, they never rebel against their Queens, perhaps due to the latter's telepathic abilities. It must be noted, however, that the Queens have no allegiance to each other. They also have developed, or stolen, advanced technology.
Their true planet of origin is unknown. They arrived in the Shi'ar galaxy long ago, and began infesting many worlds, becoming deadly enemies to the Sh'iar. In this galaxy they found certain large space-dwelling creatures that they decided to use as living starships. These include the whale-like Acanti, and the shark-like Starsharks. The Brood use a virus that effectively lobotomizes the creatures, then they use bionics to control them. The Brood hollow out part of the creatures (by eating them) and use the space created to live in, like termites eating a tree. This of course eventually kills the living ships, requiring them to capture new ones.
One of the Acanti they captured was of unusual size (its rib cage alone was the size of a mountain range.) They used it as their main base, and, when it died and crashed onto a planet, used it as their main city. (The corpse was so large, it took centuries just to rot halfway.) However, predators from the planet they landed on infested the area of the dead Acanti's brain, so the Brood avoided it.
The Brood allied themselves with the renegade Sh'iar Deathbird to help her depose her sister Lilandra as ruler of their empire; as a reward, Deathbird gave Lilandra, their allies the X-Men, and their human friend Carol Danvers (who had been the superhero Ms. Marvel before Rogue stole her powers) and even one of her own allies, Fang of the Imperial Guard, to use as hosts. The Brood did indeed make use of them, except for Danvers, apparently since they wanted to perform experiments on her half-human/half-Kree genes. The Brood created an illusion in which the heroes and their friends believed they were guests of the Shi'ar and didn't realize they were being implanted. However, Wolverine's healing power purged him of his embryo, and he helped his friends to escape. Unfortunately, he is unable to save Fang, who changes into a Brood before they leave.
The Brood Queen orders her forces to find them, until she was contacted by the Queen embryo she had implanted in Cyclops of the X-Men; it explained that the X-Men were returning to "sleazeworld". Resigned to their dooms, the heroes were trying to accomplish one last good act by helping the Acanti race to recover the racial Soul, an apparently supernatural force that must be passed from one Acanti leader ("The Prophet-Singer") to the next. The Soul was located in a crystalline part of the dead Acanti's brain. The Queen herself went with her minions to that area, and battles the X-Men until they started to change into Broods. The assistance of the alien dragon later known as Lockheed helped but did not sway the course of the battle. The Prophet-Singer Soul was almost infected by the evil of the Brood. Wolverine tries to mercy-kill his friends (and then the Queen) but they were saved when Danvers (now a star-powered being called Binary, a result of the Brood experiments on her) arrived and released the Soul. Before it went onto its next host, the soulforce cured the X-Men and Lilandra, and turned the Queen into a crystal statue. It also caused 'Sleazeworld' to explode, but the X-men and their allies, including Lockheed, escape alive. Some of the Brood also managed to escape before the planet explodes. The new Prophet-Singer then led the Acanti to safety in deep space.
A star-shark later crashes to earth, infecting several nearby humans, including one paramedic. This man (Harry Palmer) was allowed to function seemingly normally, only to be taken over when he encountered mutants, which he would then infect. This led to the first Earth-based confrontation between the X-Men and the Brood, with the latter having added the powers of several human mutants to their ranks.
Another branch of the Brood managed to reach Earth and infected several people, mainly mutants, including many from the Louisiana Thieves' Guild that Gambit belonged to. The X-Men were forced to kill most of the infected people. Fortunately, they managed to rescue many of the Brood's yet-unifected prisoners. Ghost Rider, who was helping them, became infected for some time, but that Brood aspect was destroyed.
An exception was a woman named Hannah Connover, wife of reverend William Connover of the Glory Day ministry. She was implanted with a Brood embryo by paramedic Josey Thomas, partner of Queen-host Harry Palmer, who had been infected some time before. Becoming a Brood Queen cured her hands of arthritis and she used this "miracle" to become a faith healer for her husband's ministry. She secretly infects the people who came to her for healing, curing them of their illnesses by converting them into her Broodlings. She eventually manages to overcome the Brood's control, a feat she attributed to her husband's unknowing inspiration, and rebels against the Brood Empress. The Empress sends several of her unusually powerful "firstborn" Broodlings to assassinate her lest she interfere with the invasion of Earth. The X-Men were unable to stop them in direct combat but they were ultimately defeated when she allowed Iceman to freeze her solid, stopping her brain activity and cutting her off from Brood telepathy. Having lost contact with her the assassins believed her to be dead and their mission over, so lacking a ship to return home they killed themselves. According to the time-travelling X-Man Bishop there are benign factions of Brood in the future, and he speculated that Hannah may be their originator. However, there are still malevolent Brood in the Marvel Universe.
During the Contest of Champions, the Brood abducted several heroes and posed as a benevolent species willing to give the heroes access to advanced technology after competing against each other in a series of contests. However, in reality, the Brood intended to use Rogue, who had been infested with a Brood Queen, to absorb the powers of the contest winners and become truly unstoppable. Fortunately, Iron Man (One of the first heroes to be defeated) organised the other 'losers' into a resistance against the Brood, realizing that the Brood had been feeding the heroes drugged food to amplify aggression (Iron Man's suit had automatically activated its life support system to keep him from being affected). Also, Carol Danvers and Kitty Pryde had been locked away since the Contest began, Carol's Kree physiology and Kitty's intangibility powers preventing them from being used as hosts.
Although the Queen had already absorbed the powers and skills of Captain America, Thor, the Hulk, Spider-Man, Jean Grey and the Scarlet Witch the remaining heroes managed to defeat her. The Brood Queen symbiote was subsequently extracted from Rogue with the aid of Carol Danvers, who forced the Brood Queen to flee by threatening to kill Rogue. After confirming that Rogue was cured thanks to her old link to Carol from absorbing her powers, the heroes returned home.
The Brood returned to Earth in the Ms. Marvel series and battled Carol Danvers, who as Binary played a key role in their earlier defeat. Strangely enough, none of the Brood present recognized who she was, possibly because of her apparent inability to fully access her cosmic powers, which also changed her physical appearance. The Brood were being stalked and summarily exterminated by the alien hunter called Cru, whom Ms. Marvel also came into violent contact with.
Some Brood Sleazoids appear in the arena of planet Saakar in the Planet Hulk storyline of The Incredible Hulk, one of them even becoming a main character. A Brood Queen referred to as "No-Name" apparently becomes the lover of insect king Miek and also appeared in World War Hulk. When it was discovered that Miek was the one who let the shuttle that brought Hulk to Sakaar explode, "No-Name" and Hulk attacked Miek. Near the end of the War the "Earth Hive", the shared consciousness of every insect on Earth, use Humbug as a Trojan Horse to deal a crippling blow to No-Name, rendering her infertile and poisoning the last generation of hivelings, growing into Humbug's body.