Neurotrope X was a large neurotropic virus, able to infect humans. It was visible with a microscope. Symptoms of the infection included fever, coma and, eventually, death. It was employed as a biological weapon by the Cybermen.
In 2070, on Earth's Moon, the crew of the Moonbase was infected with Neurotrope conveyed through the sugar supplies. Patients were put in quarantine. After the arrival of the Second Doctor and his party, Jamie McCrimmon was infected too. The patients were abducted by the Cybermen for cyber-conversion. (TV: The Moonbase)