Emerging viruses: Past, present, and future

"Thirty years ago, the U.S. Surgeon General declared that we had conquered disease. Unfortunately, that statement was incorrect," biologist Eddie Holmes told a rapt audience at Research Unplugged last Wednesday afternoon. The discussion focused on where viruses—such as HIV, SARS, and avian flu—originate, how they emerge in human populations, and how they evolve and survive.

Many viruses begin in animal populations and are then transmitted to humans, explained Holmes. Viruses that make this leap between species are called "zoonotic" diseases, and are the focus of Holmes' research. One example is HIV, which originated among chimpanzees in sub-Saharan Africa. In the late 1970s, with hunters increasingly encroaching into chimpanzee habitats, infected meat was introduced for sale in local marketplaces. During butchering, food preparers came in contact with chimp blood, and because human and chimpanzee DNA is nearly identical, the virus made the jump, Holmes explained to the crowd. The virus then began to spread through sexual transmission, and its impact was quickly magnified by the effects of increasing globalization.

man smiles in front of red green and blue stripes
James Collins

Eddie Holmes

Noted Holmes, humans have changed their environment drastically since the beginning of civilization. When nomadic hunter-gatherer societies transformed into farming communities, people were for the first time brought into sustained close contact with animals, creating a "breeding ground for diseases," he said.

The next major spike in the spread of viruses, Holmes noted, occurred during the Industrial Revolution, when people moving from rural areas to urban settings for factory work "brought their bugs with them." With more people living in tighter spaces, the potential for spreading viruses increased.

Today, with the emergence of mega-cities and the enormous increase in air travel, diseases surface and circulate across the globe at a phenomenal rate. "All those things put together will cause disease to occur more frequently in the future," Holmes told the crowd.

Poverty also adds to the spread of viruses. Developing countries do not have the necessary resources to combat, contain, and control outbreaks. Holmes pointed out that an individual in the United States infected with HIV might pay $5,000 to $10,000 each year for drugs to control the virus. "In sub-Saharan Africa, the health budget per person per year is just $3."

Without the costly protease inhibitor drugs that have given many with HIV a longer life span, the disease will "cut a swath through the population of Africa," Holmes predicted. However, he explained, it's not in the best interest of even the most lethal viruses to kill their "hosts" too quickly.

"Viruses need to infect new people to survive," Holmes said, "which explains why some viruses last longer than others." Deadly viruses like Ebola do not spread through large populations because the virus kills the host quickly, unlike HIV which allows the host to live with the disease long enough, sometimes unknowingly, to spread the virus, Holmes noted. And avian flu doesn't transmit well because it lives deep within the body. "It's a dead-end virus," he explained, "because, at this point, it has not generated a transmittable path from person-to-person after infection."

Holmes pointed to SARS as a "success story" in containment because within three weeks of outbreak the disease was identified and controlled, infecting only 8,000 people. One crucial weapon against future outbreaks of zoonotic disease, he suggested, is the surveying of animal populations to determine what may be on the horizon. The cost of such large-scale testing is high, he acknowledged, but monitoring is the best way to prevent the spread of viruses. No single country or organization is big enough to take on this responsibility alone, he added. "To beat a disease the world will have to work together."

Edward Holmes, Ph.D., is professor of biology. He can be reached at ech15@psu.edu.

Last Updated November 06, 2006