Posttraumatic vocal disarticulation
From Halopedia, the Halo wiki
Post-traumatic vocal disarticulation is a Human condition involving the vocal chords of a human being. After a period of extreme stress or trauma, a person diagnosed with this "disease" is rendered completely unable to speak, even if there is no catastrophic injury to their actual vocal chords. It is unclear if this condition is physical, or if it is merely psychological caused by very traumatic events. However, the words "post-traumatic" may point to it being psychological, as Lucy was one of the two other surviving SPARTAN-III personnel in her company. The other 298 were all killed in a huge explosion.
Lucy-B091 is the only known case of someone being diagnosed with it in the Halo universe
The Rookie may also have this problem since he was the lone survivor on a mission at New Jerusalem, Cygnus.
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