Critically ill patients whose lungs are supported by breathing machines (ventilators) commonly develop a new lung infection, called ventilator-associated pneumonia (VAP). Because VAP is often fatal, antibiotics are administered whenever it is suspected. However VAP is hard to distinguish from several non-infective lung conditions and most patients with suspected VAP do not have pneumonia. Therefore many patients receive unnecessary antibiotics for several days, promoting emergence of 'superbugs'. Laboratory test results for diagnosing VAP typically only reach the doctors after 3 days.