Badminton, like tennis, is a racket sport that is played by either two players or two teams of two players each. The object is for you or your team to get to 21 points first. You score a point whenever you successfully serve the shuttlecock and your opposing team commits a fault, which means that the team fails to appropriately return the shuttlecock.
Assuming one cares for their teeth by brushing regularly, saliva may smell bad because there are millions of bacteria present in the mouth. Bad smelling saliva or breath can be indicative of a tooth abscess, or a systemic illness such as diabetes.
Many sufferers complain of a continuous metallic taste in their mouth. This is from the iron in the blood which produces the metallic taste. With the blood loss, the breath can become worse. First it'll have an iron or metallic smell and then change into a more foul scent as the blood decomposes. While checking for bad breath, check to see if there's inflammation of the gums and blood. This will be a good indicator that the bad breath is due to gum disease. Reference: Mouth Health: Bad Breath.