Can animals tell when you're sick? I think they can. My dog seems totally able to tell when to take it easy on me. On my bad days, he stops at the top of the stairs and waits for me before he starts walking down. Then he waits at the bottom patiently while I catch up (the down is always harder, isn't it?)
I see the difference in how he acts when my husband is holding the leash instead of me. He runs farther, strains against the leash, chasing every smell, every sound. There's no difference in the level of happiness, he seems just as content to trot along with me as he is to bound around with my husband.
Whenever I'm sick, like infection/virus kind of sick, my cat will lay right on top of me. She lays right on my rib cage like she's trying to protect me. Any time I feel kind of gross and she lays on top of me, I go and check my temperature and sure enough, it'll be up over 100. So basically, my animals can tell when I'm sick before I can tell that I'm sick.
And people say that animals aren't smart.