Mitral valve stenosis can arise following an infection with a bacteria called streptococcus. This bacteria is associated to strep throat and scarlet fever. If either of these infections are untreated they can lead to a serious condition called rheumatic fever which is an inflammatory/immune condition which affects the heart and joints.
Now a days rheumatic fever not as common as it used to be thanks to tests that lead to the early recognition of the bacteria and the prompt use of antibiotics. However, rheumatic fever is still prominent in underdeveloped countries.
Other causes of Mitral Valve stenosis include cancers, radiation to the chest (radiation heart disease), and conditions which cause calcium to lay onto valves.