I listened to an audio book a few years ago that contained a story about a boy who started seeing another version of himself in his room.
At first he only saw the other, but as the story progressed the two gained the ability to communicate verbally. Eventually, the boy was able to shift over into that second universe even though he could not interact with it.
Meanwhile, interspersed throughout, were news stories in the other universe about a newly discovered needle-shaped biting insect. Turns out that the insect conferred an infection that produced more insects and even larger alien creatures within the people it bit. Near the end of the story we find out that the initial infection is tiny, but the aliens quickly develop a critical mass that rubs against and weakens the fabric of reality.
At the very end, the boy from universe one finds himself in the second universe where he is infected. When he shifts back to his home universe, the infection is carried over.