Short for 甲型肝炎, "type-A hepatitis". The Heavenly Stems 甲, 乙 and 丙 serve as A, B and C, giving 甲肝, 乙肝 and 丙肝.
history
In early 1988 about 300,000 people in Shanghai caught hepatitis A after eating blood clams (毛蚶) that were raw or only briefly scalded, one of the largest outbreaks on record.