Subject cohorts: Cohort 1: Stable IPF group (n>150); Cohort 2: AE-IPF group (n>50); Cohort 3: healthy control group (n>100). All the IPF patients have been enrolled according to relevant diagnosis criteria. The criteria of AE-IPF include: diagnosed IPF patient experiences unexplained dyspnea within 1 month, with objective evidence of hypoxia and new onset of pulmonary infiltration based on imaging examination, and with other diagnosis like pulmonary embolism, pneumothorax or heart failure excluded.Serial observation of the change of pathogens based on at least two times of sample collection. Healthy control group recruits healthy population based on physical exam, with no definite respiratory disease and normal chest X ray results.