medicala chronic illness or chronic pain is serious and lasts for a long time. A serious illness or pain that lasts only for a short time is described as acute(疾病)慢性的;(疼痛)长期的
a nervous illness or medical condition is caused by worry, or by working too hard, so that you feel excited when you want to relax (疾病)由精神紧张引起的,由精神压力造成的
medicalan opportunistic disease or infection is one that attacks people who are already ill and who have a very weak immune system(疾病或传染病)机会性的,感染免疫力低的病人的