Aspergillus is a fungus which occurs everywhere in the world and normally lives on dead animal or plant material, in this role it is vitally important to the environment and for the recycling of organic material necessary for life.
It produces microscopically small spores that are extremely light and float easily in the air and by this mechanism it is spread and propagated.
More details - with a multi-lingual translation.

Normally, when aspergillus spores are inhaled by people, their immune system recognises the spores as foreign and they are destroyed and no infection arises. 

Occasionally, in an individual with a weakened immune system or who has a pre-existing medical condition the aspergillus spores can grow inside a lung or a wound. 
This type of  infection caused by aspergillus is termed aspergillosis.

Aspergillosis describes a number of diseases involving both infection by and growth of this fungus, as well as allergic responses against aspergillus. Aspergillosis can occur in a variety of body organs, in both humans and animals. Infections can effect any area of the body, but by far the most common are the lungs and sinuses.