8. Allergies

When you have the flu, you might have noticed your breath smells kind of bad. This is because respiratory tract infections like bronchitis, sinusitis, and the flu can trigger the production of bacteria-feeding cells and mucus due to the inflammation of respiratory tissues.

Mucus buildup will clog noses, making it hard to breath. Similar to sleep apnea, this will make people breathe through their mouths, resulting in dry mouth and an increased production of foul-smelling bacteria.