I truly do believe that my cat can definitely tell when I am sick or emotionally drained. Almost always when I am relaxing myself, he's always around to cuddle and rest with me. But naturally, when I am not sick or mentally drained, he hardly cuddles! (He's not necessarily a cuddly cat!) So I definitely agree that cats do know when you are sick.
There has been times where I've been in so much pain from being sick or other health issues and my cat would run from downstairs and immediately come upstairs to cuddle with me. Especially when I complain to myself that I can't do something. It's almost like he thinks that he is the answer to all of my problems -- which obviously he is!
