While it's not their preferred tactic, Reaper nanites can "infest" a living body, repurposing it to their own ends by replacing its structures with Reaper equipment. This "infection" is caused by the nanites themselves. As such it's airborne, waterborne, bloodborne, can be transmitted through material as the Reapers eat through it. It's a world killer. The only thing that can stop it would be things that destroy Reaper nanites- EMP being the primary one.

It progresses extremely quickly, from the first symptom (numbness in the extremities) to full maturity in about two days. As it progresses, black spots appear in the skin, followed by connected conduits and plates as the victim becomes increasingly biomechanical, and finally mechanical. Mercifully, Reaper infection is fairly painless, and as it progresses it leads to coma, and eventually death of the host as the last of the brain tissues are replaced.

Reaper infection can be cured with EMP, but it's a huge ordeal to fix the changes- there's examples of "survivors" who've even benefited some from it. The first thing Reapers do is fix any imperfections in their host, to make the reconstruction easier. Also, if you let it go long enough, you get sweet four-fingered claws and a spooky blue Reaper glow from your insides.

It's really a last resort for a swarm who REALLY NEEDS a larger body but doesn't have the materials. In normal circumstances, they'd prefer to gobble something down entirely and build their own host engine.