Fungi cause diseases in humans in different ways. Due to production of secondary metabolites by molds, dematiaceous hyphomycetes, and primary plant pathogenic fungus species intoxications are included in their pathogenic potency. Mycotoxins mostly enter the body by ingestion of food because they are contaminants of plant products and in case of carry over also of animal products. Recent investigations have shown that mycotoxins detected in feed and food during microbial deterioration can also be produced inside warm-blooded organism, e. g. with aspergillosis in animals (mammals and birds). This means an enhanced risk for humans to suffer from mycotoxicosis, in which the toxic agent has an exogenic or endogenic source. Therefore, attention should be paid to the fact that mycotoxins are not only damaging cells; they very often possess a genotoxic potential.