Hi there,
Bad breath is caused due to build up of bacteria in your mouth that causes inflammation and gives off noxious odors or gases that smell like sulfur .The most frequent cause is plaque left on the teeth that has formed calculus, tooth decay ,cavities, gum disease, acid reflux, respiratory tract infections, chronic bronchitis and post nasal drip. Lack of oral hygiene during fever and lack of brushing your teeth regularly can lead to bad breath from the mouth. If the symptoms are severe or persistent seek medical help. I hope it helps.
Best luck and warm regards!