The fungus Aspergillus fumigatus is everywhere, and is extremely dangerous for people with weakened immune systems. It occurs virtually everywhere on Earth, as a dark grey stain on damp walls or in microscopically small spores that blow through the air and cling to wallpaper, mattresses and floors. Healthy people usually have no problem if spores find their way into their body, as their immune defence system will protect them. However, the fungus can threaten the lives of people with a compromised immune system, such as AIDS patients or people who are immunosuppressed following an organ transplantation.