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。

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。

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。

The reason, they say, is simple: "a sick dog is as much a dependant as a sick child."

他们说这样做的理由很简单：“狗儿生病就像孩子生病一样需要人照顾。”