Bad breath also is known as Halitosis.
Maybe due to improper brushing, underlying gum disease, Diet, improper cleaning of the tongue. It may also be due to any underlying Health disease like diabetes, sinus infection, lung disease, Gastrointestinal problems etc. 

How can you prevent bad odor?
Proper brushing, flossing, cleaning your tongue is important as most of the time the bad breath is due to not cleaning of your tongue. Mouth rinsing after taking every meal. Keep your self-hydrated and avoid drying of your mouth.

Visit your dentist for the treatment of any gum diseases.