Bad breath can also be called halitosis. The most common cause is a build up of bacteria in the mouth; we all get these bacteria, but if you don't brush your teeth regularly they can build up and form plaque. This can have serious effects including tooth decay, but in the short term it also causes bad breath. This can be prevented by careful brushing, use of dental floss or interdental brushes, and if necessary, mouthwash.

Another cause is eating very strongly flavoured foods such as garlic. Alcohol can also leave an unpleasant smell, especially next day, and most non-smokers find the smell of a smoker's breath quite off-putting. The problem with smoking is not just the immediate smoke smell, but additional long-term bad breath as smoking increases the build-up of bacteria mentioned above.

There are also certain medications which cause halitosis, and in this case it can be harder to deal with. You can learn more about the causes and treatments here.