What causes alpha hemolytic streptococcus?

That is a good question! In beta hemolysis there is colorless zone around the colony of bacteria. There is complete lysis of the red blood cells. This indicates the virulence of the bacteria. In alfa hemolysis, there is green coloration around the bacterial colony. This indicate the partial virulence. Gamma hemolysis means no destruction of the red blood cells. So streptococcus viridens can affect the caries tooth and damaged valve only. By the way caries tooth cause mild rheumatism like the rheumatic heart disease. You can cure the patient of rheumatism by taking out the caries tooth / teeth.