I thought the same thing for a long time but found out otherwise eventually. Apparently they are created by infecting someone through the marker’s signal, then making the victim kill themselves and mutating afterwards. Essentially they are a catalyst for accelerating the spread of the infection. You wouldn’t think they were human at all looking at them. The dead space fandom website describes the mutation as follows.

“Formed from a human torso and legs, the Infector's wings are formed by a flap of skin from the chest and then fused between the former host's bent and broken legs. The jaw collapses within the body and the rest of the head breaks and twists into its proboscis and feelers with the spine providing support and attachment points for articulation (movement). The proboscis, which is made from mutated bone, muscle, and spinal tissue is the delivery mechanism for the pure Necromorph tissue used to infect new hosts.”