Disclaimer: Fruits Basket is the property of Natsuki Takaya. The following is a work of fanfiction loosely based on that property. This work is unofficial.
Kyo was 6 years old when he first tasted them. He was eating a dinner with Kazuma—takeout because the man could not cook—comprised of overcooked fish and pan-fried negi. Kyo was excited about the fish, but the negi looked decidedly unappetizing.
"They're good for you," he was told. Kyo put one in his mouth, and chewed thoughtfully for a moment; he decided fairly quickly that negi did not taste very good, and said as much. Eventually, he grudgingly ate a small portion of them because he was a good boy.
The following morning Kyo fell ill.
The sun was shining but Kyo was in a daze; he felt like it was raining outside, despite the fact that he could see sunlight through his window. But Kyo didn't feel well at all, had little appetite, and felt unnaturally tired.
Some part of Kyo didn't want to stay in bed all day; bed was boring. But he wasn't certain he wanted to get up either (and when he really admitted it to himself he didn't really feel like doing anything) and that bothered him. Eventually things became bad enough that Kazuma found an orange cat lying almost unmoving in his adopted son's bed.
Eventually, through diet and rest, Kyo recovered and reverted back to human form. From that point onward the boy associated the unpleasant taste of the vegetable with illness.
And after the color finally returned to his vision, Kyo told himself that he wouldn't eat leeks ever again.
Author's note: Onions and garlic are bad too.