Windchime essentially said the people in the immortal universe have a fixed destiny. If their destiny is at all tampered with, they get RDS. To put it simply Marie and her fiancée were not supposed to get married or maybe even meet. Because they did though, not only them, but potentially 2 or more people got infected with RDS as well. The inverse could also be true. Because Marie's "soulmate's" destiny was altered she got RDS along with them and that principle would apply to her fiancée as well. Again, not just Marie and her fiancée, but potentially 2 or more people got RDS with them and they didn't do a thing. RDS doesn't behave like a disease, rather it's a long, elaborate domino effect with dominoes getting knocked down at random points in the chain reaction.