Yaka are disease-causing spirits in Sri Lankan myth. Their name is derived from the Yakshas and Yaksinis in India. They are ruled by their king, Wakahola Sanni Yaka, to whom shamans make numerous offerings.
"Demons of disease in Sri Lankan mythology. Masks of them are made so that shamans can talk with them. They can be summoned and exorcised through ritual dancing while wearing the mask."
—Shin Megami Tensei III: Nocturne Compendium
Shin Megami Tensei IMAGINE
"A type of disease-causing demon found in Sri Lankan legends. They originated from the Yaksa and Yaksini of India. Yakkha are small and incredibly ugly in appearance. Those who fall under their gaze are afflicted by disease and madness. Almost all diseases in Sri Lanka are said to be caused by Yakkha, and must be cured by a shaman or witch doctor. To do so, the healer presents an offering and performs a dance to appease the demon and make it leave. The Yakkha are ruled by their king, Mahakola Sanni Yakkha, to whom Sri Lankan shamans and witch doctors make numerous offerings. If a Yakkha will not leave a person's body, the healer will call upon the king himself to force the demon to leave."
—Shin Megami Tensei IMAGINE Compendium
Yakka can be contracted on Ichigaya field, in Kagurazaka Zhu Que Cavern's bronze-level instance, and most cars in the Old Tokyo Metro.
Devil Summoner: Soul Hackers
"Demons of sickness in Sri Lankan mythology, said to originate from the Yaksha of Hindu mythology. They bring illness and insanity to humans. They have the power to transform, and their original form is small and ugly."