Researchers have now discovered how the fungus -- Aspergillus fumigatus -- knocks out the immune defences enabling a potentially fatal fungal infection to develop. Among other factors it is gliotoxin -a potent mycotoxin - that is responsible for the pathogenicity of Aspergillus fumigatus. Pathogenicity refers to the ability of an organism to cause disease. It was known that this substance has an immunosuppressive effect which means that it weakens the activity of cells of the immune defence system. However it had not been clear previously how exactly this happens said Oliver Werz Professor at the University of Jena in Germany.