Aspergillus is a common fungus found throughout the world, spread by microscopic spores which float in the air. We all breathe them in most of the time but most of us have efficient immune systems that kill the spores.

Some people do not manage to kill all of the spores; sometimes because they are undergoing treatment that suppresses their immune systems (e.g. transplant patients, bone marrow transplant recipients), but sometimes because they are genetically susceptible (e.g. some asthma sufferers) or possibly because they have breathed in many more spores than is usual.
They develop aspergillus infections, known as aspergillosis.

There are several types of aspergillosis which we go into in some detail here, but briefly there is:

Allergic Bronchopulmonary Aspergillosis (ABPA)
You have a fungal infection deep down in your lungs. This irritates the lung tissue and causes scarring. The infection is treated with antifungal medication and the inflammation and scarring is reduced using steroids. More...

Chronic Pulmonary Aspergillosis (CPA)
Your lungs have been damaged in some way in the past (e.g. tuberculosis) and the fungus can grow in the affected area. Sometimes it grows as a fungal ball (i.e. aspergilloma) in the cavity. Sometimes several cavities are involved (CCPA). More...

Invasive Aspergillosis
Almost exclusively affects very highly immune suppressed people e.g. transplant patients, bone marrow transplant recipients. Must be treated very urgently which is why this group of patients are retained in hospital during the danger period of their treatment. More...

Aspergillus Sinusitis
Chronic aspergillus infection of the sinuses. Still controversial but an increasingly accepted diagnosis. More...