Zoltán Frid was a vampire. When he met Priskin, he was shot several times but wasn't effected by him. He infected one of Priskin's men but it didn't work properly. He spent his time trying to work out what had gone wrong. Frid was patient zero of the gestalt vampiric entity. As part of his plans, he desired the blood of Anita Kereki to get information about Budapest.
In 1980, the Fourth Doctor theorised that Frid wasn't a true vampire but was a similar being. He could decide who would become like him or not. Frid told the Doctor that he used to extract information from prisoners during the Middle Ages and that he was locked up so the secrets wouldn't get out. When he picked up radio signals, he decided to become active again as there was more information for him to obtain. He planned to sire the Doctor and then infect the whole universe, turning everyone into a vampire.
The Doctor placed a hypnotic suggestion in Celia Soames's mind so that when Frid took information from her mind, the Doctor could use it against him. The suggestion was that if Frid heard the word "sunbathing," he would walk into the sun and die. This happened and his victims recovered. (AUDIO: The Labyrinth of Buda Castle)