Did someone say ‘crazy theories’? Just say my name.
Working backwards Justine was infected by the Winter Lady, who was infected by Lea, who was infected by Cowl with the Athame, who was infected by Elaine who is Kumori, who was infected by Justin when he died, who was infected by Kemmler who may have been patient zero. Kemmler, Cowl and Maeve were working with Nemesis like Nick with Anduriel learning what they could and passing it on - Justin, Justine, and Elaine were not doing so voluntarily making them less than ideal hosts. This presumes Nemesis needs a host, and can transfer only to someone in their proximity.
The Summer Lady was never infected, she just went crazy like Maeve, but had to work around her mantle rather being able to break its hold.