What Are Emerging Infectious Diseases?
Emerging infectious diseases are infections that have recently appeared within a population or those whose incidence or geographic range is rapidly increasing or threatens to increase in the near future. Emerging infections can be caused by:
- Previously undetected or unknown infectious agents
- Known agents that have spread to new geographic locations or new populations
- Previously known agents whose role in specific diseases has previously gone unrecognized.
- Re-emergence of agents whose incidence of disease had significantly declined in the past, but whose incidence of disease has reappeared. This class of diseases is known as re-emerging infectious diseases.
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.
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. For example, warming temperatures allow mosquitoes - and the diseases they transmit - to expand their range into regions where they previously have not been found.
A factor that is especially important in the re-emergence of diseases is antimicrobial resistance - the acquired resistance of pathogens to antimicrobial medications such as antibiotics. Bacteria, viruses, and other microorganisms can change over time and develop a resistance to the drugs used to treat diseases caused by the pathogens. Therefore, drugs that were effective in the past are no longer useful in controlling disease.
Another factor that can cause a disease to re-emerge is a decline in vaccine coverage, so that even when a safe and effective vaccine exists, a growing number of people choose not to become vaccinated. This has been a particular problem with the measles vaccine. Measles, a highly contagious and serious infection that was eliminated from the U.S. in 2000 and from the Western Hemisphere in 2016, has returned in certain areas due to an increase in the number of people opting to take nonmedical vaccine exemptions for reasons of personal and philosophical belief. This has been driven by an anti-vaccine movement that was founded largely on an invalid and discredited study that claimed a link between a vaccine against measles and autism. As a result of the decline in vaccine coverage, measles cases are highest by far this decade with more than 1,000 cases of measles reported in the U.S. in the first half of 2019.
Research on Emerging Diseases
The development of vaccines and antimicrobial drugs and the remarkable eradication of smallpox had created hope that infectious diseases could be controlled or even eliminated. However, the current realization that infectious diseases continue to emerge and re-emerge (including the possibility of bioterrorism), underscores the challenges ahead in infectious disease research.
To help meet this challenge, research is ongoing in the Department of Molecular Virology and Microbiology at Baylor College of Medicine on a number of emerging and re-emerging diseases, including influenza, SARS and MERS, dengue, chikungunya, Zika, tuberculosis, and HIV/AIDS. This work encompasses both basic research in trying to understand more thoroughly how these agents cause disease and how the human immune system responds to these infections, as well as more directed research in developing and evaluating vaccines and other tools to prevent infection by these agents. In addition, scientists are studying mechanisms by which bacteria can acquire antibiotic resistance and ways to combat drug-resistant infections.