The Continual Challenge of Emerging Infectious Diseases

Emerging infectious diseases, which have shaped the course of humanity and caused incalculable suffering and death, will continue to confront society in unpredictable ways as long as humans and microbes co-exist, write authors from the National Institute of Allergy and Infectious Diseases (NIAID) of the National Institutes of Health in a review article published in the July 8 issue of the journal Nature.

In their paper, the authors classify three types of emerging infections and consider methods for their control: newly emerging infections (e.g. HIV, SARS); re-emerging/resurging infections (e.g. influenza, West Nile virus); and deliberately emerging infections (e.g. microbes used for bioterror).

The authors note that emerging infectious diseases are superimposed on a constant backdrop of established infections. Approximately 15 million deaths in 2002 were directly attributable to infections, according to the World Health Organization. Tragically, the authors point out, the burden of all infections falls most heavily on those least able to manage them: people living in developing countries, especially infants and children, and indigenous and disadvantaged minorities in developed countries.

Why do infectious diseases emerge and re-emerge? The viruses, bacteria and parasites that cause these diseases continually and sometimes dramatically change over time. The authors note that emergence results from “…dynamic interactions between rapidly evolving infectious agents and changes in the environment and in host behavior that provide such agents with favorable new ecological niches.” As a result, new pathogens arise, and familiar ones re-emerge with new properties or in unfamiliar settings.

Historically, the authors write, the results have been devastating. For example, importation of smallpox into Central America caused 10–15 million deaths in 1520–1521, effectively ending Aztec civilization. AIDS, first recognized in 1981, now threatens to surpass in global fatality the “Black Death” of the 14th century and the influenza pandemic of 1918–1919, two notable infections that emerged to each kill tens of millions of people.

In the past five years alone, two pathogens well known to countries on other continents were seen in the Unites States for the first time—West Nile virus and monkeypox virus. In addition, a new infectious disease, SARS, emerged in 2003 and has since caused more than 8,000 cases of illness and nearly 800 deaths around the world. In addition, in 2001 the United States was confronted with a third, extremely disquieting category of threat: a disease resulting from the deliberate release of an infectious agent, anthrax, by a terrorist(s).

The authors write that an effective response to any new infectious disease threat, whether it emerges, re-emerges, or is deliberately introduced, involves mobilizing many different types of public health activities. In particular, frontline surveillance and response is critical and depends on rapid detection, clinical diagnosis and containment. Concomitantly, basic and applied research enables the development of medical countermeasures such as surveillance tools, diagnostic tests, vaccines and therapeutics. The authors note that these efforts have been accelerated by advances in fields such as genomics/proteomics, nanotechnology, direct and computational structural determination, immunology, and geographical information systems and satellite imaging.

Reference: DM Morens, GK Folkers and AS Fauci. The challenge of emerging and re-emerging infectious diseases. Nature 430: July 8, 2004.