I have only read Ender's Game and the Speaker for the Dead, and did not yet read the other books from the series. So it might be already explained in other books. But I could not find the explanation from the Speaker for the Dead.
It is explained in the book, that the Descolada is reason the Starways Congress could not evacuate the Lusitania Colony. The Xenobiologists say that if anyone from Lusitania goes to another planet, the Descolada agent would destroy all the ecosystem, and they would not have the time to make all the animals and plants resistant to the disease.
But, in the last chapter, Miro decides go to Trondheim, aboard Ender's starship, and take the trial. Why would Miro do that? And why would Ender agree with this?