Methods Patients Between July 1, 2010, and June 30, 2012, we will enroll about 280 patients with mechanical ventilation for more than 21 days and no evident signs of infection on admission to our Respiratory Care Centers (RCCs) for clinical study. The study has been sent to the Ethics Committees of China Medical University Hospital for approval. Patients or guardians provide written informed consent after our explanation. Exclusion criteria were informed consent lacking/refused, ongoing antibiotics treatment for infection, predicted mortality in 7 days. All patients will be followed up until death or discharge from our hospital.