Been ill with mild flu to 5 days now, foul taste in mouth --->?

Several things. Could be from post nasal drip down the back of your throat causing a foul taste in your mouth. If you have upper tooth pain, face pain, along with drainage that is discolored from your mouth or nose seek medical attention. Brush your teeth often with a good toothbrush that is not old. You might want to soak your toothbrush in an antibacterial mouthwash overnight too.