What is the image or ending message Preston leaves us with?

The author reflects on the ease in which a virus like Marburg or Ebola can spread. It is an unsettling feeling he leaves us with. He goes back to the Kitum Cave and the Monkey house to see nature reclaiming the places but this time he wonders if viruses are simply part of nature. He wonders if viruses like AIDS is a price paid for destroying natural habitats or simply another natural mechanism for controlling population.