Ganados are humans implanted with and controlled by Plagas, parasites that were formerly confined beneath Salazar Castle. While the humans implanted with Plagas lose the ability to reason independently once the parasite has latched onto their central nervous system, they retain their intelligence and are able to communicate with and understand one another. The type of Ganado that develops after implantation depends on the human host—these particular Ganados are adherents of the cult of Los Illuminados.
Ganados are intelligent, which clearly distinguishes them from Zombies, who obey only the most rudimentary of survival instincts. Being screamed at in a human language—even one that's incomprehensible—results in a kind of fear that's entirely different from that which is experienced when facing Zombies. Incidentally, the Ganados speak in Spanish, and all of their lines actually mean something—finding out what can be an enlightening experience to say the least...