count the number of patients that are diagnosed with int inf clstrdium dfcile until 3 years ago.
select count( distinct admissions.subject_id ) from admissions where admissions.hadm_id in ( select diagnoses_icd.hadm_id from diagnoses_icd where diagnoses_icd.icd9_code = ( select d_icd_diagnoses.icd9_code from d_icd_diagnoses where d_icd_diagnoses.short_title = 'int inf clstrdium dfcile' ) and datetime(diagnoses_icd.charttime) <= datetime(current_time,'-3 year') )