Sick as a "Dog"??

Meaning: very sick. Simile dating back to the 1500s or earlier.

Where do we, English speakers, come up with these idioms? Personally I've never really seen a very sick dog.
What happens when a dog really is sick? Do we say: "Poor thing, he's as sick as a human, a hominid, a Homo sapiens, an Adamite"?

All good questions to ponder when you are stuck at home sick as a dog. :)