When the Europeans colonized the New World, they transmitted many different diseases to the Indigenous Americans. This led to massive depopulation of the Western Hemisphere that the Europeans exploited.
ROB changes the diseases that the Europeans transmit. Whenever a Native American got sick from a European or African disease, instead of the usual symptoms like smallpox, they instead get a fever for a couple of days and then they die. After they're dead (even if they didn't die of the fever), they reanimate as a shambling zombie similar to The Walking Dead. If the zombie bites someone, they gain the virus. Old Worlders can only gain the virus through zombie bites. Zombies can somehow function despite decomposition or missing limbs. Zombies need a destroyed brain to fully die. If a zombie doesn't find someone or something (animals) to eat after 10 years, it dies.
How would the colonization of the Americas be different? Will all Native American corpses be cremated or mutilated now? Could a pandemic start in Europe and Africa? Will the Europeans write off the Americas as cursed and leave it uncolonized for a long time? Can any of the native polities survive in this timeline? Could this virus be cured or exterminated?