This research is about whether treatment with a commonly used antibiotic can prevent infections in airway and lungs and improves the chance of surviving, if it is given soon after patients commence mechanical ventilation when they have been admitted to hospital with an acute severe brain injury. An acute severe brain injury can occur as a result of a stroke, a traumatic injury or due to lack of oxygen to the brain that happens as a result of a cardiac arrest. Patients who are unconscious after an acute severe brain injury often need assistance to breath adequately, and this assistance is given by a breathing tube, connected to a mechanical ventilator. This treatment is an emergency medical treatment. The breathing tube is inserted into the patients' airway by either their mouth or neck. For patients who need assistance with their breathing from a mechanical ventilator, infections in the airways and lungs, known as pneumonia, are a common complication. Everyone naturally has bacteria in their mouth, esophagus and stomach. Clinicians think that during the process of inserting the breathing tube, small amounts of these bacteria can be introduced into the airways and lung when people are unconscious following an acute severe brain injury, or during the process of placing the breathing tube into the airways. These bacteria are now in a place they aren't meant to be and can cause an infections in the airways and lungs known as pneumonia. The purpose of this research is to see if giving one dose of a common antibiotic can prevent patients developing pneumonia, which is associated with having a breathing tube inserted and being on a ventilator, improving the chance of recovery following the acute severe brain injury and ultimately improving the chance of surviving. When patients have a known infection, current guidelines are to treat them with antibiotics. Antibiotics work to kill the bacteria causing the infection. When a patient has an infection in their lungs, they often need to stay on the mechanical ventilator for longer. While current practice is to give patients with a proven infection in their airways and lungs (pneumonia) antibiotics, it is unknown if giving an antibiotic to patients to prevent these infections before they show signs of pneumonia may lead to better outcomes.