The word 疾病 is formed by combining two characters, 疾 and 病, which both individually mean "illness" or "sickness". This is a common pattern in Chinese where two synonymous or closely related characters are paired to create a single, more comprehensive or emphatic concept. 疾 often carries a connotation of a swift or acute ailment, while 病 suggests a more serious or chronic condition. Together, they form a holistic term that encompasses the general idea of "disease" in its broad sense, strengthening and completing the meaning through their semantic union.