fever 发烧，发热
eg: Ihave a fever, so I don't go to school today.
我发烧了，所以今天不去上学。

toothache 牙疼
eg:
---I have a toothache.
---Oh, I am sorry to hear that.
我牙疼。
我深表遗憾。

stomachache 胃疼
eg:I eat something unfit, SO I have a stomachache now.
我吃了不卫生的东西，所以现在胃疼了。

see a doctor 看医生
eg: You need to see a doctor.
你需要去看看医生。