Factors in the Emergence or Re-emergence of Infectious Diseases

There are many factors involved in the emergence of new infectious diseases or the re-emergence of “old” infectious diseases. Some result from natural processes such as the evolution of pathogens over time, but many are a result of human behavior and practices. Consider how the interaction between the human population and our environment has changed, especially in the last century. Factors that have contributed to these changes are population growth, migration from rural areas to cities, international air travel, poverty, wars, and destructive ecological changes due to economic development and land use.
For an emerging disease to become established at least two events have to occur – (1) the infectious agent has to be introduced into a vulnerable population and (2) the agent has to have the ability to spread readily from person-to-person and cause disease. The infection also has to be able to sustain itself within the population, that is more and more people continue to become infected.
Many emerging diseases arise when infectious agents in animals are passed to humans (referred to as zoonoses). As the human population expands in number and into new geographical regions, the possibility that humans will come into close contact with animal species that are potential hosts of an infectious agent increases. When that factor is combined with increases in human density and mobility, it is easy to see that this combination poses a serious threat to human health.
Another factor that is especially important in the re-emergence of diseases is the acquired resistance of pathogens to antimicrobial medications such as antibiotics. Both bacteria and viruses can change over time and develop a resistance to these drugs, so that drugs that were effective in controlling disease in the past are no longer useful.
Climate change is increasingly becoming a concern as a factor in the emergence of infectious diseases. As Earth's climate warms and habitats are altered, diseases can spread into new geographic areas. In fact, this has already happened. For the first time, a tropical disease has caused an outbreak in Europe. In late summer of 2007, more than 100 residents of the town of Ravenna, Italy suffered from a mysterious disease that produced fever, exhaustion, and severe bone pain. The outbreak was eventually shown to be caused by chikungunya virus, a relative of the virus that causes Dengue fever, previously found in tropical regions around the Indian Ocean. Due to warming and globalization, the tiger mosquito which transmits chikungunya virus has been able to move north and thrive in areas across southern Europe and spread the chikungunya virus. Although chikungunya virus does not usually cause a fatal disease, this outbreak serves as a warning that other, more devastating tropical diseases could follow.