Those who caught it were to fall asleep and live forever within the world of their own dreams. It is spread from person to person by spoken words, but before he could learn more, the disease took him as well.
NieR and Grimoire Weiss have shown not to be immune as they have found themselves stuck in the Mayor's dream upon entering the Forest of Myth. They found the triggering words to their infection to be "Those who dream"
They found that the only solution was to enter a dream and solving a set of riddles before leading them to find their own exit