There are several expressions of the form sick as a ..., that date from the Eighteenth and Nineteenth centuries. Sick as a dog is actually the oldest of them, recorded from 1705; it is probably no more than an attempt to give force to a strongly worded statement of physical unhappiness. It was attached to a dog, I would guess, because dogs often seem to have been linked to things considered unpleasant or undesirable; down the years they have had an incredibly bad press, linguistically speaking (think of dog tired, dog in the manger, dog’s breakfast, go to the dogs, dog Latin — big dictionaries have long entries about all the ways that dog has been used in a negative sense).
At various times cats, rats and horses have been also dragged in to the expression, though an odd thing is that horses can’t vomit; one Nineteenth-Century writer did suggest that this version was used “when a person is exceedingly sick without vomiting”. The strangest member of the set was used by Jonathan Swift in 1731: “Poor Miss, she’s sick as a Cushion, she wants nothing but stuffing” (stop laughing at the back).
Er...anyway. When we left for the dentist on Wednesday, I had a cough. Thought nothing of it. I never get sick. I mean, not germy sick. By the time we got back to Atlanta that evening, I had a sore throat. By Thursday night, I was achy and running a fever of 100F, which feels like 101F for me, because I'm a freak and my normal body temperature is actually 97.7F (Spooky, also a freak, averages 97.3F). And since then, I have bounced feverishly up and down, mostly slept, moaned, bitched, cursed microbes, and annoyed Hubero with my mountains of snot rags. Too woozy to sit up for very long, I have not written. This is officially my worst "flu/cold" sort of crud since 2003. Anyway, at some point, Spooky took the following humiliating photograph:
Erg. Here comes gravity again. Maybe by tomorrow I'll only feel like ass.