Andronesian encephalitis was an unpleasant, but not life-threatening, disease transmitted by airborne particles that was common in Federation space due to the transmission of the particles via space travel.
In 2152, when Phlox insisted on examining Travis Mayweather, he justified this with the possibility that Mayweather could suffer a seizure at the helm due to Andronesian encephalitis. (ENT: "Singularity")
Although the disease was easily treated and regarded as a minor condition by 2365, Anya, the guardian of the Dauphin of Daled IV, regarded it as a threat and requested that the person infected be terminated. (TNG: "The Dauphin")