病 (bìng) can be a noun ('illness, disease') or a verb ('to be ill'). As a verb it means simply 'to be sick', while 生病 (shēng bìng) emphasizes the act of falling ill. As a noun, 病 is the everyday word for any sickness, whereas 疾病 (jí bìng) is more formal and often used for diseases in a medical context.