Aphasia was a perceptual dysfunction characterized by loss of or deficit to someone's ability to use, generate, participate in, and/or understand language, usually caused by some sort of damage to the brain's language centers, or – less frequently – by damage to other areas of the brain.
An aphasia virus was left behind on Terok Nor following the Cardassian occupation. Miles O'Brien was one of the first to be infected there by this virus after accidentally triggering it in 2369. (DS9: "Babel")
In 2375, Seven of Nine, who believed she was suffering from Albright-Salzman syndrome, was diagnosed with sensory aphasia. Ducane informed her that it affected the cerebral cortex and could cause temporal psychosis. (VOY: "Relativity")