What is the tone of the last sentence of the book? Why does Preston choose to end the book this way?

Ebola had risen in these rooms, flashed its color, fed, and subsided into the forest. It will be back.

I actually chose to put the last two lines because the emphasis is on the fact that for now the hot zone has gone away and the natural jungle is intruding again. The Ebola took its place in "the hot zone," it showed itself, and then it returned to the forest. But noting that "it will be back" notes that the argument is not over.