Bronchiolitis is the commonest cause of hospital admission in infants. It is caused by Respiratory Syncytial Virus(RSV), a virus that causes mild 'common colds' in adults, but can cause lung inflammation and difficulty breathing in infants and the elderly. In 2005, nearly 34 million cases in children <5 years occurred, resulting in 3.4 million hospital admissions,and around 120,000 deaths. There is no vaccine, and some previous vaccines actually made the disease worse.