Spore carriers are humans who had been exposed to an entomopathogenic fungus, known as B. mordicana. It was developed to colonize the bodies of common pests. Verified in a report in a terminal entry in the medical wing, the host technically dies once the fungus has fully colonized the host body, but the fungal colonies continue to animate the body. In this manner the fungus moves amongst more of its prey, occasionally spraying spores in a radius around the host body, infecting all who come near it. Its drawbacks include the time it takes to kill prey (10–20 days), and its limited effectiveness when dealing with nonsocial pests.
The B. mordicana fungus was developed before the Great War in the Big MT research facility, and was delivered to Vault 22 as a part of the Vault Behavioral Project. Its birthplace is the X-22 botanical garden, which eventually itself succumbed to the plant life. The inhabitants of Vault 22 were exposed, and those of them who left the vault have spread the fungus as far as Zion Canyon, where spore carriers pose a problem even today. It seems that the spread of the green fungus happens over time, as the doctor who first performed an autopsy noticed nothing wrong externally with the first infected victim.
Spore carriers are quick and use melee attacks. Spore carriers share some characteristics with the trogs from the Fallout 3 add-on The Pitt. If they are not killed quickly, they explode and deal large amounts of damage, breaking limbs, and giving a large dose of radiation. When they are hiding in vegetation, they are hard to spot and will not appear on the Courier's compass, nor be targeted via V.A.T.S. until they move.
Walking past unexplored vegetation can result in an unpleasant scare. Spore carriers are also known for sneaking up on their victim. However, their health meter will become visible if they are injured by an area-of-effect weapon, like a flamer or grenade, immediately revealing their presence.
Upon detecting an intruder, the spore carrier will raise themselves out of the large patch of vegetation where they hide, and will let off a growl in a similar manner to that of a feral ghoul, but with greater ferocity.
While a spore carrier is getting up or sleeping, attackers in sneak mode who manually attack the spore carrier will get a critical. Combined with a decently powerful weapon, such preemptive strikes can often kill the spore carrier before it can cause any harm.
Spore carrier brutes are found in the same areas other spore carriers are found, and usually tend to act the same. However, brutes tend to be more aggressive, and are tougher and larger than regular spore carriers.
Very small versions known as "runts" can be found in Vault 22. This helps put into play the horror of what happened in the vault, even to infants and small children.
The spore carrier appears to be a re-skin of the trogs from The Pitt, albeit with spores on its back. They are very similar to the trogs because of their category names (brute and savage), appearance, animation and troglodyte nature.
If the Courier survives a carrier's explosion, they will be rewarded with a significantly increased amount of experience.
If the reticule is held over a dormant carrier, it will be identified as non-hostile or most normally they will not even show anything on the reticule, as if it were part of the grass.
A report on a terminal in the medical wing in Vault 22 verifies that the spore continues to grow on the host after the host dies.
When close to a spore carrier while it is still "sleeping," it can be interacted with. However, it will only note that "spore carrier is unconscious."
If their "head" is targeted, which is only the top part of the head, shooting this in V.A.T.S. will sometimes result in the top part exploding, instead of the entire head like most other enemies or characters, akin to the super mutant overlords and behemoths of Fallout 3.
When killed, spore carriers make the same sound as a feral ghoul makes upon death.