Introduction
The European Society of Clinical Microbiology and Infectious Diseases (ESCMID) and European Confederation of Medical Mycology (ECMM) wanted to tackle a challenge that no major scientific society had tried: providing a guideline on the diagnosis and management of rare and emerging fungal diseases. This guideline would obviously exclude Candida and Aspergillus diseases. Practically all invasive fungal diseases (IFD), including invasive candidiasis and aspergillosis, appear to be rare and emerging infections by definition. Although many IFD are still numerically rare, physicians treating immunosuppressed patients are increasingly confronted with a wide variety of fungal pathogens. Rarity of disease is defined by their absolute frequency in a population, and definitions range around 1 in 2000. Of course these statistics are different for populations of severely ill patients, where frequencies of IFD are much higher.