The answer to the question is given in the previous book in the series, "Speaker for the Dead ". When Ender arrives on Lusitania he is told that the descolada is kept in check by ingesting a dietary supplement - the Colador - developed by Novinha's parents, Gusto and Cida:
"Nonsense," said the Bishop. "Your parents found a cure for that."
"They didn't cure it," said Novinha. "They controlled it. They stopped
it from becoming active." "That's right," said Bosquinha. "That's why
we put the additives in the water. The Colador."
They unfortunately died of the descolada before enough of the drug could be produced, but their results were not secret, and the use of the supplement is known generally throughout the colony:
...we have to eat a supplement, like an extra vitamin, to keep the
plague from striking again. They told you that, didn't they? If you
once get it in your system, you'll have to keep that supplement all
your life, even if you leave here.
So it is not a case of Novinha surreptitiously "lacing" the food. What is secret is that the descolada adapts to the supplement, and periodically it has to be changed when the descolada becomes immune to it. As time went on, the rate at which the descolada adapted increased. But while Novinha was growing up, the first form that her parents developed was sufficient, so the colony was able to survive without a Xenobiologist.