Emerging Infectious Diseases
Introduction to 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.
The World Health Organization warned in its 2007 report that infectious diseases are emerging at a rate that has not been seen before. Since the 1970s, about 40 infectious diseases have been discovered, including SARS, MERS, Ebola, avian flu, and swine flu. With people traveling much more frequently and far greater distances than in the past, the potential for emerging infectious diseases to spread rapidly and cause global epidemics is a major concern.
Additionally, there is the potential for diseases to emerge as a result of deliberate introduction into human, animal, or plant populations for terrorist purposes. These diseases are discussed under Bioterrorism Agents.
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.
One example of an emerging infectious disease that can be attributed to human practices is HIV. It is thought that humans were first infected with HIV through close contact with chimpanzees, perhaps through bushmeat hunting, in isolated regions of Africa. It is likely that HIV then spread from rural regions into cities and then internationally through air travel. Further factors in human behavior, such as intravenous drug use, sexual transmission, and transfer of blood products before the disease was recognized, aided the rapid and extensive spread of HIV.
Influenza (or flu) is an example of an emerging disease that is due to both natural and human factors. Influenza virus is infamous for its ability to change its genetic information. Large changes in the influenza virus can cause pandemics because the human immune system is not prepared to recognize and defend against the new variant. The chances of large genetic changes occurring and being passed into humans are increased when humans coexist in close proximity with agricultural animals such as chickens, ducks, and pigs. These animals are natural hosts of influenza virus and can act as mixing vessels to create novel versions of influenza that have not existed previously.
Avian H5N1 influenza (or bird flu), which emerged more than a decade ago, has been limited to relatively rare instances of infection in humans who came into direct contact with diseased birds. The H5N1 virus is very deadly (more than half the cases have been fatal), but it has not acquired the ability to pass efficiently between humans. In contrast, the 2009 H1N1 influenza, which passed into humans from swine (pigs), transmitted easily from person to person. The H1N1 virus traveled around the world faster than any virus in history as a result of human activity, particularly air travel. Fortunately, it was much less deadly than the H5N1 virus. Emergence of an influenza virus that is as deadly as the avian H5N1 virus and is spread between people as easily as the swine H1N1 virus would be a very serious threat to human health.
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 ( MVM) at Baylor College of Medicine ( BCM) on a number of emerging and re-emerging diseases, including influenza, Ebola, tuberculosis, dengue, 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, and more directed research in developing and evaluating vaccines and other tools to prevent infection by these agents.
Click on a specific topic in the menu bar at the left or at the bottom of this page for general information about the agents that cause these diseases and specific research projects being performed by MVM investigators in these areas.
For more information:
http://www.niaid.nih.gov/dmid/eid/ - Listing and information about emerging and reemerging infectious diseases from the National Institutes of Health
http://www.cdc.gov/ncidod/diseases/eid/index.htm - CDC National Center for Infectious Disease website
http://www.cdc.gov/ncidod/diseases/eid/disease_sites.htm - Information about specific emerging or reemerging infectious diseases from the CDC
http://www.who.int/topics/emerging_diseases/en/ - Information about emerging diseases from the World Health Organization
http://www.cdc.gov/ncidod/eid/index.htm - Journal devoted to Emerging Infectious Diseases