Why do I always feel weak when I am sick?

I currently have a really bad cold, and I was carrying some things up the stairs. When I got to the top, I had to sit and breathe for a minute before I could keep going, even though I normally wouldn’t have any trouble at all. After making a few trips carrying things upstairs, I was completely exhausted. As I pondered this, I realized that I am always ridiculously weak when I am sick. What causes this?