Aphasia is the loss of the ability to communicate effectively.
Aphasia was a medical side effect of using the Guardian of Forever. In 2294, a civilian Leonard McCoy wished one of his New York patients would complain of this to liven up the day. (TOS novel: The Fearful Summons)
During the Occupation of Bajor, two scientists of the Bajoran Resistance were able to create a viral form of the disorder. They placed a sample on Terok Nor, hoping to infect the Cardassians, although it did not activate.
In 2369, Miles O'Brien accidentally activated the device and an epidemic spread throughout Deep Space 9. While Julian Bashir attempted to find a cure on the station, Kira Nerys sought out Dr. Surmak Ren, one of the device's creators. Dr. Surmak assisted Bashir in finding a cure. (DS9 episode: "Babel")