What Could Cause Pre Diagnosed Polymyositis?

My friend of over 50 years (Female, age 72) has been pre-diagnosed the polymyositis. No cause can be thought of; however, she had salmenilla (spelling?) a year ago from unwashed lettuce at a well-known restaurant and this year had a bout of bedbugs in a home she was caregiving. Blood was found on her sheets. Could this situation contribute to this diagnosis?