The Descolada is the virus that infects everything on Lusitania. It turns piggies into trees, watersnakes into the weeds by the river, and humans into dead humans… until Novinha's parents find a cure. The Descolada produces miraculous strangeness and new variety, or else death, and as such, it neatly captures the novel's enthusiasm for, and wariness of, foreignness and difference.
The Descolada made the new world, and when it gets in the humans it changes them too. To deal with it, to interact with strangeness, the colonists need to work hard and with care. They need to use science, knowledge, and skill to figure it out and keep it in bounds. Otherwise they end up like Pipo's daughter Mary, "a new limb, not arm or leg, growing out of her hip, while the flesh sloughed off her feet and head, baring the bone" (1.32). That's a horror movie image of change as nightmare, and it stands in stark contrast to the still painful but joyous transformation of Human—"out of his spine a sprout burst upward, two leaves, four leaves—" (17.465). The Descolada is the alien inside as destroyer, as thing to be tamed so we can stay the same, and (since Human has to stand for humans) also as potential transformation.