Natural disasters happen whether the environment has been altered or not. Tsunamis, volcanoes, hurricanes and earthquakes hit different areas often. If the area lacks buildings, the damage is much less severe. The flying debris of human creations causes more damage than the falling of trees and changing of landscape. In addition, most naturally occurring structures such as mountains and plants grow in areas where the living organisms have adapted to survive. Trees grow around rocks; animals have the ability to hole up and then come out and survive. Humans have a more difficult time when man-made objects are destroyed and they need to find food and water to sustain life.
Truly natural environments provide a fair and equal balance for life. This does not mean the areas are suitable for human living but rather the animals that are meant to inhabit the area. Most natural environments have been irrevocably altered by people. The introduction of foreign plant and animal life changes the balance of nature. The removal or killing of certain animals and plants can also have an impact. In Yellowstone, the deer population grew out of control once all of the wolves were removed from the area -- so much so that in 1996 the wolves were reintroduced to the area to try to decrease the number of deer.
Natural environments do not provide for the general comfort humans have come to expect and desire. Truly natural environments do not provide a place for waste, electricity or easy transportation. This limits the amount of trade and support you can get from the outside world. You must be able to provide all the food to sustain life. Communications do not exist in a natural environment due to the lack of electricity and wiring.
Disease runs rampant in truly natural environments. Human digestive tracts have adjusted to purified water. The microorganisms existing in natural surroundings can make most people sick and even cause death. Without the ability to bring in or produce certain medications and vaccinations, humans become more susceptible to dangerous diseases. Within a few generations, you could see the resurgence of seemingly dead illnesses such as polio and measles.