Many children and young people are at risk of invasive fungal disease (IFD), such as those who have had a haematopoietic stem cell transplants, those with an immune deficiency or those who are prescribed immunosuppressive drugs, for example, corticosteroids. One type of mould that causes invasive fungal disease is called Aspergillus. There is currently no quick test which can tell us if someone has an invasive fungal disease caused by Aspergillus called Aspergillosis. It is a difficult condition to diagnose and the results from the tests that are involved take days or weeks to come back. These tests including a few different blood tests, a scan of the lungs (CT scans) and taking fluid from inside the lungs/airway. A new test for Aspergillosis is the lateral flow device (LFD) assay. This is a rapid test which gives a result within minutes. It involves testing a sample of the fluid from the lungs/airway. This fluid can be obtained as part of the routine investigations for Aspergillosis. It has been shown to be a good and safe test in adults but the investigators do not know if it will be a valuable test in children and young people yet. The purpose of this study is to determine whether the LFD test can effectively diagnose Aspergillosis in children and young people.