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. The patients were abducted by the Telosian Cybermen for cyber-conversion. (TV: The Moonbase)