Crippled

Summary
A Crippled is created when an important neural connection of the parasite isn't present or destroyed. This can happen either through disadvantageous mutations in the metamorphosis or reproduction of the larvae, or through the use of chemical substances or physical force on the Infected.

Most Crippled are characterized by their inability to move and use certain parts of their bodies, which is obvious from the visible rot and decay of the body part in question. Many cases of crippling lead to the eventual demise of the infected by lack of nutrition, while other Crippled can last for extended periods of time when situated in ideal conditions.

Still, because of their disabilities, they aren't considered a threat by humans, and only the most incautious and cocky survivors fall victim to these largely harmless creatures.

Value to the ecosystem
A Crippled can, however, still act beneficially in regard to the entire infected ecosystem. Most, if not all Crippled which are rendered stationary take it upon themselves to act as a "beacon", uttering unhuman noises and alerting other nearby infected to the presence of potential prey.

This regard for the other infected doesn't seem to be present when the Crippled is still able to move.