This study is a prospective, randomized, double-blind, placebo- controlled clinical trial examining whether oral supplementation with bLF can reduce nosocomial infection, sepsis and even mortality in patients with prolonged mechanical ventilation (MV). Patients with MV for more than 21 days and no signs of infection on admission to our Respiratory Care Center (RCC) will be enrolled. They will be randomized to receive either bovine LF (bLF, 10 mg/kg/day) or placebo for 6 weeks by center.