Sick as a What?

As we've noted previously, I've been a bit ill for the past few days. The phrase "sick as a dog" reverberates through my head. And yet, I live with two dogs. Kriston's rat-killer and Spencer's floor-urinator and I have to say that they don't seem to get sick ever. I mean, obviously I know that dogs develop serious health problems and eventually die, but they seem relatively free of maladies like the flu or the common cold. I can even see why this might be: You tend not to see large numbers of dogs congregating in close quarters, dogs never stay out drinking later than they should for a few nights in a row and gut their immune system, dogs tend to maintain a nice, stable diet, etc. Plus dogs -- even very well-loved dogs -- don't usually get nearly the level of medical attention that we give to people so the evolutionary pressures toward general good health are more serious.

So then: Where does this phrase come from? Also, I swear to God that when I quit smoking I was promised fewer respiratory ailments. And I only rarely even had any respiratory ailments. I feel that the medical establishment has really betrayed me here.