He supposed it was a side-effect of the 14th's infestation of his body, or maybe it was his health failing generally, but either way he knew he couldn't tell anyone. Half because he knew Lenalee would worry herself half to death (as she has a habit of doing) and half because he knew the Order would restrict him even more; possibly even removing him completely from duty.
So he suffered in silence, suppressing the rising panic with increasing difficulty.
Honestly, it wasn't much of a difference really.
He didn't even notice it until he came back from a surprisingly simple Innocence retrieval mission. He'd gone to see Jeryy to get a solid meal in him and found Lavi annoying Kanda in the dining hall and discovered that Lavi's usual bright red hair was a rather dull shade. Thinking nothing of it (or rather, blaming it on his exhaustion) he ate, joked and went to bed. In the morning it had gotten even worse. Lavi's red hair now looked more like old, bleached paint. In fact, all colors looked washed out yet darker, as if someone was slowly adding a filter to his vision.
Maybe he hadn't noticed it while he was away on the mission (it may have been easy but he traveled for weeks, maybe even months because apparently it was as far away as it possible. Of course.) but now that he had noticed it it was as if it suddenly progressed over night. The rational part of his brain said that it hadn't, but it still terrified him.
He went about with his life, did his duty, was scrutinized by the higher-ups and grew even closer to his friends. In his spare time, when he was sure no one was watching him, he'd read anything he could get his hands on to figure out what was going on but he came up empty.
There wasn't much he knew but there was one thing he could conclude for sure: Allen Walker was going blind, and he could never tell anyone. Ever.