Zytron energy was a type of energy that could be used to power objects. It was cheap to use and very adaptable, but also unstable and hard to contain. It was also highly mutagenic, causing cellular disruption if the containment failed or leaked. When suffering from Zytron energy poisoning, individuals would start to develop darkened lumps all over their body, their eyes would lighten in colour and they increased in size and strength. They also started to become more aggressive until they could't think properly any more. These traits would increase in severity until the victim died.
It was discovered by humans in 4947 and used by the Jal Karath before the 1940s. When Darac-7 came to India, his ship started leaking Zytron energy. In less than a week, people started mutating from it. The Doctor was able to remove the Zytron energy after Darac-7 was defeated, curing the afflicted people. (PROSE: Ghosts of India)