In an increasingly interdependent world, the United States faces an array of new global challenges that transcend the traditional definition of national security. One important example is the resurgence of infectious disease. In the 1960s and 1970s, powerful antibiotic drugs and vaccines appeared to have banished the major plagues from the industrialized world, leading to a mood of complacency and the neglect of programs for disease surveillance and prevention. Over the past few decades, however, infectious diseases have returned with a vengeance.

Worldwide, 20 well-known maladies, including tuberculosis, malaria, and cholera, have reemerged since 1973 in more virulent or drug-resistant forms or have spread geographically. Over the same period, at least 30 previously unknown diseases have been identified for which no cures exist. Examples include Ebola and other hemorrhagic fevers in Africa, the worldwide AIDS pandemic, Legionnaire's disease, Lyme disease, Hepatitis C, "mad cow disease," Sin Nombre virus, Nipah virus, and new strains of influenza. Although AIDS was not recognized until the 1980s, it now infects some 36 million people worldwide and kills three million annually. Since 1980, the US death rate from infectious diseases has increased by about 4.8 percent per year, compared with an annual decrease of 2.3 percent in the 15 years before 1980. At present, nearly 170,000 US citizens die annually of AIDS and other infections.

Not only do importations of disease threaten US citizens directly, but devastating epidemics such as AIDS are spawning widespread political instability and civil conflict in countries where the United States has significant interests. The Clinton administration sought to address these challenges by placing public health on a "new security agenda" along with other nontraditional threats such as environmental degradation, dwindling supplies of clean water, global warming, mass migrations of refugees, and overpopulation. In response to critics who questioned the relevance of infectious disease to national security, President Clinton's National Security Advisor Sandy Berger wrote recently, "[A] problem that kills huge numbers, crosses borders, and threatens to destabilize whole regions is the very definition of a national security threat.... To dismiss it as a 'soft' issue is to be blind to hard realities."

In January 2000, the National Intelligence Council supported the Clinton administration's policy by publishing an unclassified National Intelligence Estimate (NIE) assessing the implications of the global spread of infectious diseases for US national security. Packed with sobering statistics, the NIE concludes that over the next 20 years, emerging and re-emerging infectious diseases will endanger US citizens at home and abroad, threaten troops deployed overseas, and exacerbate political and social instability in key countries and regions. This instability, in turn, will contribute to humanitarian emergencies and military conflicts to which the United States may have to respond. The report also warns that the threat of biowarfare and bioterrorism will grow as rogue states and terrorist groups exploit the ease of global travel and communication to pursue their deadly objectives.

Multiple factors have facilitated the emergence and spread of infectious diseases. The overuse of antibiotics to enhance the growth of chickens and cattle has contributed to a dramatic increase in drug-resistant microbes at a time when the discovery of new antibiotics has lagged; the settlement of formerly remote jungle areas has brought humans into increased contact with exotic viruses; the rise of megacities in developing countries with poor health systems has created "hot spots" for the evolution of new infectious agents; climate change has led to a shift in the geographical distribution of pathogens and their insect vectors; and the growing volume of cross-border travel and trade associated with globalization has provided new opportunities for microbial "hitchhikers." For example, the West Nile virus, which caused a major outbreak of encephalitis in New York City in the summer of 1999, had never been seen before in the Western Hemisphere and may have been imported by an infected traveler, a migrating bird, or a stray mosquito on an airplane. Viruses also have the potential to mutate into more lethal and contagious forms, as occurred with the Spanish Flu, which killed more than 20 million people around the globe during 1918-1919 and could well strike again.

The NIE develops three alternative scenarios for the global impact of infectious diseases over the next 20 years. An optimistic scenario projects steady progress toward controlling infectious diseases, while a pessimistic scenario foresees little or no progress as AIDS spreads through the vast populations of India, China, the former Soviet Union, and Latin America, and multiple-drug-resistant strains emerge at a faster pace than new drugs and vaccines can be developed. The third, most likely scenario begins with a decade of deterioration as the AIDS pandemic becomes more severe, followed by limited improvement owing to better prevention and control of childhood diseases, new drugs and vaccines, and gradual socioeconomic development.

In the short term, the NIE predicts that, in the hardest-hit countries of the developing and former communist worlds, the persistent burden of infectious disease is likely to aggravate and even provoke economic decay, social fragmentation, and political polarization. Already, the collapse of public health systems in Russia and the former Soviet republics has led to a dramatic rise in HIV infection and drug-resistant tuberculosis in those countries. By 2010, AIDS and malaria combined will reduce the gross domestic products of several sub-Saharan African countries by 20 percent or more, bringing these nations to the brink of economic collapse as they lose the most productive segment of their populations.

If current trends continue, a decade from now some 41.6 million children in 27 countries will have lost one or both parents to AIDS, creating a "lost generation" of orphans with little hope of education or employment. These young people may become marginalized or easily exploited for political ends, as in the increasingly pervasive phenomenon of the child-soldier, putting AIDS-stricken countries at risk of further economic decay, increased crime, and political instability. The NIE suggests that by the year 2020, AIDS and tuberculosis will account for the overwhelming majority of infectious disease deaths in the developing world. Nevertheless, a somewhat more hopeful picture has emerged in recent months as growing political pressure has led multinational pharmaceutical companies to lower the price of AIDS drugs sold to poor countries.