What illness had Tsunku recovered from after losing his vocal cords?

Tsunku had recovered from laryngeal cancer after losing his vocal cords during treatment.

Laryngeal cancer affects the larynx, the organ in the throat that contains the vocal cords. Tsunku announced that he had the disease in 2014, and in 2015 revealed that his vocal cords had been removed as part of his treatment.

Tsunku is a central figure in the Rhythm Heaven series: he has served as a producer and has composed music throughout the franchise. Rhythm Heaven Groove’s article notes that he worked on the game without his vocal cords while recovering from the illness.

This fact is sometimes misstated as thyroid cancer or as a respiratory illness such as bronchitis or pneumonia. Those conditions do not match the illness identified in the game's development history; the canonical answer is laryngeal cancer.