"It has to do with the mitochondria. They need to be able to process it. Not sure why, but you see there are a bunch of different mitochondria types, but they pass through the mother line. The genetic compatibility is because you and your sister have the same mitochondria, but you have different DNA. The Mitochondria...." The Doctor trailed off with wide eyes. "I can save them all. I can make the mitochondria fight off the Torx, that's why they can't just connect to humans, and maybe even why the Hyiasam doesn't affect you." 

The Doctor spun around pleased with herself. "You're brilliant." She hugged Nora and then dashed about the TARDIS again. "I can do this!" She slammed the tracking machine on the consul and hooked a cord to it. She typed furiously and then the tracking machine made some funny noises. "Nora! We can save everyone!" She bounced up and down, holding the tracking machine proudly. 

"Alright here's the plan, those guys come back and I start sending this signal out. You get their attention. Keep them coming toward me. We can do this." The Doctor took Nora's hand and walked out of the TARDIS with her.