That's what one of my students asked me yesterday.
---What makes you think I'm sick? Is it the fact that I can't speak more then 4 sentences without coughing, or I have to blow my nose during the spelling pre-test? Did I tip you off that I lost my voice during the read aloud? I can't talk loud enough to talk over you, so I sit by the clock and count seconds until you notice that I'm trying to get your attention. I guess you noticed because I have been squeezing my head, because of a headache, more than normal.
---So why don't we have a sub, you ask. Because dear 6th grader, If I were to get a sub, I would have to plan out many more lessons, make many more copies, grade many more papers then usual. I would also have to deal with more behavior and respect problems, other teachers giving me looks for skipping out on my duties, and a sub that leaves a report telling me you're all hyper, disrespectful, and can't stay on task. Does that sound like something you want to do when you're not feeling well?
--So I'm here at school today, don't worry you won't catch my cold. You probably gave it to me. I know how to cover my cough, wash my hands, and not share food. I can keep my germs to myself. So please talk a little less, listen a little more, and grab a tissue for me.