Iron is a necessary cofactor for many enzymes, and can act as a catalyst in the electron transport system. "A. fumigatus" has two mechanisms for the uptake of iron, reductive iron acquisition and siderophore-mediated. Reductive iron acquisition includes conversion of iron from the ferric (Fe+3) to the ferrous (Fe+2) state and subsequent uptake via FtrA, an iron permease. Targeted mutation of the ftrA gene did not induce a decrease in virulence in the murine model of "A. fumigatus" invasion. In contrast, targeted mutation of sidA, the first gene in the siderophore biosynthesis pathway, proved siderophore-mediated iron uptake to be essential for virulence. Mutation of the downstream siderophore biosynthesis genes sidC, sidD, sidF and sidG resulted in strains of "A. fumigatus" with similar decreases in virulence. These mechanisms of iron uptake appear to work in parallel and both are upregulated in response to iron starvation.