My 8 month old son was diagnosed with mastitis. how could he get this? does this happen often?

Not very often: Mastitis is inflammation and infection of the breast tissue. Commonly happen from contamination from own skin, staph or strep are the most common bugs. Its also associated with micro trauma or contusion/ compression of breast tissue plus blockage on the tiny pipes of the gland . Sometimes required just warm compress and oral antibiotics. Occasionally incision and drainage by the surgeon