Perspectives in Health Magazine
The Magazine of the Pan American Health Organization
Special Centennial Edition
Volume 7, Number 2, 2002

Another "recently emerged" virus, HIV (human immunodeficiency virus), illustrates a different process. The simian version was a chronic infector of chimpanzees in Africa before crossing species and infecting humans, causing acquired immune deficiency syndrome, or AIDS. Transmitted primarily by sexual intercourse, unsterilized needles and blood transfusions, AIDS has spread rapidly worldwide by capitalizing on its chronic nature and the long period during which carriers show no symptoms but can transmit the virus. In spite of its stealth strategy, AIDS claims the lives of virtually everyone it infects within a decade or so. Some will be able to stave off its lethal effects by taking expensive and toxic medications, but the majority of the 40 million people infected today will die without even this benefit. In all likelihood, HIV will be the most significant infectious agent of the 21st century barring the emergence of something even worse.

A more agile offender is the influenza A virus. We tend to think of influenza as a minor illness that comes to torment us every winter. But this is only partially true. The virus circulates in humans and gradually changes its covering proteins to escape protective responses from our immune system.

Independent of this slow strain drift is another process that occurs outside humans, in which many different influenza strains randomly exchange their genes. When a highly virulent and transmissible combination occurs and makes its way into the human population, a worldwide epidemic, or pandemic, results. The virus spreads quickly because no one has any significant immunity to the novel strain, and the illness it causes can be unusually severe. The worst pandemic was in 1918, when more than 20 million people died worldwide from a particularly virulent influenza strain. The last was the 1968-69 "Hong Kong flu," which killed 700,000. Since flu pandemics tend to occur every 10 to 40 years, we are statistically overdue for a new one. In 1997 we had a very close call: a virus appeared in domestic fowl in Hong Kong and killed nearly one-third of the people subsequently infected. Fortunately, it proved to be of limited transmissibility between humans, and prompt surveillance and disposal of infected birds eliminated the threat.

What would happen in a new pandemic? It is questionable whether we could detect a new virus in time to prepare a new vaccine in quantity. We might turn to antiviral drugs, but stocks are low, and expanding them rapidly would not be feasible. The death toll from a new pandemic might be only a million or so or it could rival or exceed the 20 million deaths in 1918.

Ecological change
The driving forces behind these problems are in great part related to population growth and human behavior. Satellite images show that people have modified more than half the world's surface, leading to profound changes in land use, water availability and even climate. As agriculture occupies progressively larger zones, flora, fauna and associated infectious disease agents are shifting rapidly. The movement of species from one ecosystem to another in these disrupted ecological zones can result in an overrun by a new invader.

In North America, we think of this most frequently with plants such as kudzu and dandelions, but many animals were also introduced. The most dangerous of these are the ubiquitous Rattus norvegicus (the urban gray rat) and Mus musculus (the domestic house mouse), which were introduced into the Americas from the Old World, bringing with them plague, rickettsialpox, Seoul virus and lymphocytic choriomeningitis virus. Aedes aegypti, the mosquito vector of dengue and yellow fever, was also introduced into the Americas, carrying those viruses and making possible their spread across the entire Region. More recently, in 1999, West Nile virus was introduced, probably via a stowaway mosquito on an airplane, into New York from the Middle East. Dr. Robert Tesh, at the University of Texas Medical Branch in Galveston, recently isolated the virus from a Houston blue jay, confirming its spread as far west as Texas. Nationwide, the virus has caused 149 human cases, including 18 deaths, and will probably reach Central and South America in the near future.

By far the most common underlying causes of emergence can be found in changes in the ecology of the infectious disease or its hosts. A classic example is Bolivian hemorrhagic fever, which emerged in the 1960s. Machupo virus, which causes this dangerous disease, is carried by rodents, and settlements in new areas of Bolivia's department of Beni led to its emergence as a concentrated human health problem. The cause of the disease was unknown, but it invaded small towns, causing high death rates and often prompting the abandonment of entire settlements. When authorities discovered that the disease was caused by a rodent-borne virus, the animals were eliminated and the towns were safe again. However, rodents continue to live in clearings where isolated houses are built and in fields where crops are planted to sustain nearby towns. The well-fed animals in the fields remain a constant threat to the rural people who work there.

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