29 Physical changes combined with cultural and economic factors to spread Ebola. Evolution and natural selection at the molecular level also contribute to the spread of disease. Changes in the genetic composition of humans and other animal and plant species can force parasitic organisms to change, sometimes altering the delicate balance between hosts and infectious microbes. A disease can emerge or re-emerge because its causative microbe has evolved into a more infectious form. A microbe's ability to infect the host can also change if the genetic makeup of host populations has evolved, usually through minor mutations of genes, to be more susceptible to the pathogen.
Environmental change is an important force in the emergence and re-emergence of infectious disease.
The human penchant for manipulating the environment is a double-edged sword. In the past two centuries, humans have actively raised living standards, improved health status, reduced infant and child mortality, and lengthened life expectancy. Manufactured shelters, especially those with controlled heating and cooling, helped slow the spread of IPDs. And, the arsenal of medical weapons humans created dramatically reduced many diseases, and eliminated one-smallpox. But the same human innovations that brought improved health and increased longevity also created conditions that favor the re-emergence and more rapid spread of IPDs.