From AIDS to SARS to MERS, Emerging Infectious Diseases Remain a Dire Threat

New viruses like the pathogen that causes MERS are jumping across the species barrier, going from animals to human beings. Can we even eliminate infectious disease?

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Passengers on a subway train in Hong Kong in the spring of 2003, during the outbreak of SARS

About a week and a half ago, I was lucky enough to be walking through San Francisco’s Golden Gate Park on a purely perfect Northern California summer day. I was thinking what I usually think whenever I visit the Bay Area: “Why don’t I live here?” (The BART strike that began the following day gave me my answer.) But then I wandered into a secluded section of the park that I’d never visited before. The National AIDS Memorial Grove is a shelter of trees and stone that was created in 1991 and nationalized in 1996, and at first glance it simply looks like another beautiful part of one of the most beautiful urban parks in America. But then you notice the memorial stones scattered around the grove that are carved with remembrances of friends and family lost to AIDS. “Dedicated to Douglas Watson and Larry Silva who met the day humans walked on the moon.” “Send in the clowns for Steve Sem.” This grove is the most permanent memorial the U.S. has to an epidemic that has already taken the lives of more than 600,000 Americans and more than 30 million people worldwide.

When HIV/AIDS first began to emerge widely in the early 1980s, we weren’t too far removed from an era when doctors believed humanity was on the verge of essentially wiping out infectious disease. In 1969, U.S. Surgeon General William Stewart declared “we can now close the book on infectious diseases,” and when the world managed in 1980 to eradicate smallpox — a disease that killed as many as 500 million people in the 20th century alone — that confidence seemed warranted. HIV — a virus that initially jumped from chimpanzees to humans more than a century ago — shattered that illusion fast, as did the new infections that would emerge in the years to follow: Nipah virus, SARS, H5N1 avian influenza, hantavirus pulmonary syndrome. The natural world was still more than capable of producing new pathogens capable of killing human beings.

Now the world is facing another emerging infectious disease. MERS — Middle East respiratory syndrome — is in the same family of coronaviruses as SARS, which killed at least 775 people after it emerged in China in late 2002. MERS, which first appeared in Saudi Arabia in September, has been kicking around the Middle East for nearly a year, infecting at least 79 people. It causes fever, cough and shortness of breath, and so far it has been a killer — about half the confirmed cases so far have resulted in death. On July 9 the World Health Organization (WHO) convened an emergency meeting to determine whether the new coronavirus that causes MERS constitutes a “public-health emergency of international concern,” as WHO assistant director general Dr. Keiji Fukuda put it. (For more about MERS, read the WHO’s latest update.)

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The good news is that a recent report published in the Lancet indicates that the virus has a relatively low level of infectiousness — less so than the measles and strong cases of the flu — which may limit its potential to ignite a global pandemic. A similar lack of infectiousness also kept SARS from becoming a lasting global menace, though the disease did cause nearly $50 billion in damages. But there’s no guarantee that MERS won’t mutate or worsen over time. And even if it doesn’t, there will always be new infectious diseases waiting to emerge, as Drs. David Morens and Anthony Fauci warn in a new paper:

While it has become possible to eradicate certain infectious diseases [smallpox and the veterinary disease rinderpest], and to significantly control many others [dracunculiasis, measles, and polio, among others], it seems unlikely that we will eliminate most emerging infectious diseases in the foreseeable future. Pathogenic microorganisms can undergo rapid genetic changes, leading to new phenotypic properties that take advantage of changing host and environmental opportunities. Influenza viruses serve as a good example of emerging and re-emerging infectious agents in their ability to rapidly evolve in response to changing host and environmental circumstances via multiple genetic mechanisms. New ‘founder’ influenza viruses appear periodically, cause a pandemic, raise widespread population immunity, and then, in response to human immune pressures, evolve and persist for decades using multiple genetic evolutionary mechanisms to sustain continual immune escape. The 1918 influenza pandemic virus is one example: over the past 95 years, its descendants have evolved continually by antigenic drift, intrasubtypic reassortment, and antigenic shift, the latter producing new pandemics in 1957 and 1968. Even the genetically complex 2009 pandemic H1N1 influenza virus is a descendant of the 1918 virus. Such continuous genetic hyperevolution forces us to develop new influenza vaccines containing new antigens on an annual basis.

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Morens and Fauci — the latter of whom was on the front lines of the battle against AIDS in the early 1980s — track the threat of both emerging and re-emerging infectious diseases. Dengue and West Nile viruses are two of the latter. Dengue first emerged in Africa centuries ago, but was brought over to the Americas thanks to the slave trade, most likely through infected slaves who seeded the mosquito population in North America when they arrived. (Both dengue and West Nile are transmitted via mosquitoes.) Changing health conditions allow old diseases to become more dangerous — because of the immune suppression that results from HIV infection, fungal diseases like cryptococcal meningitis, which a healthy person would be able to fight off, have become leading causes of death in HIV hot spots like sub-Saharan Africa.

The dream of eliminating infectious disease is dead. The global community has spent billions of dollars to try to finally stamp out polio, but that disease has proved stubborn. And the growth of globalization has given infectious disease a boost. As we push into wild places like the rainforests of central Africa, human beings come into contact with exotic species with exotic germs. Air travel — which grows each year — puts nearly every corner of the planet, no matter how remote, within a day of a major city. Population growth of both people and domestic animals like chickens and pigs means all the more fuel for new microbes to feast on, and makes it easier for viruses to leap across the species barrier. If we’re lucky, MERS will be another viral dead end, not contagious enough to do lasting global damage. But as experts like Morens and Fauci know, we won’t be lucky forever.

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