Flu is a contagious respiratory illness caused by influenza viruses that could infect the nose, throat and lungs. An annual flu vaccine can be the best way to prevent flu infections. A new test has been developed to identify influenza patients who will require urgent, life-saving, medical treatment. The High-risk Influenza Screen Test (HIST) can help to measure ''an early warning signal'' that is released by the patient''s body to their blood for the immune system to fight the infection.