When I get sick, my teeth become disgustingly yellow. Why is that?

Oral hygiene. There are a couple of things that may causse this. First when you are sick you may not be brushing your teeth as much as you usually do so the backteria can build up on your teeth and cause them to look yellower. The bacteria in you mouth will change when you aer sick, so try to brush and floss regularly. If you are dehydrated then you teeth can be effected and look a bit darker.