How do you say illness in Japanese?
— 病気 (びょうき, byōki); 疾病 (しっぺい, shippei)

Specialized synonyms for illness in Japanese
disease病気 (びょうき, byōki); 疾病 (しっぺい, shippei)
Derivative terms for illness in Japanese
ill病気 (びょうき, byōki)
sick病気 (びょうき, byōki)