If you are sick you could say, I feel as sick as a dog.

如果你真的不好受的话，你可以说我病得象条狗一样。

youdao

I am going to the doctor for a check-up, I feel as sick as a dog.

我想去医生那里做个检查，我觉得自己病得很严重。

youdao

Again, he looked 13 as sick as a dog and explained that earlier that morning, he'd been struck with terrible back and stomach pains.

他又一次看上去很虚弱，他解释说，那天一早，他的背和肚子都痛得很厉害。

youdao

And,the situation would be even worse if they became sick as a dog.

VOA: special.2010.05.30