Infectious Diseases, Once Thought Near Eradication, Show a Remarkable Persistence
Almost 20 years ago, smallpox was eradicated from the face of the Earth, and medical science was feeling bullish. With good reason: as James W. Brown and Elin A. Gursky wrote recently in the Medical Laboratory Reporter, at that time “it appeared civilization’s oldest enemy—infectious disease—had been beaten at last.” After all, if “one of the worst scourges of mankind” could be eliminated, “couldn’t other deadly pathogens be eradicated from the world globally?”
Unfortunately, that optimism seems sadly misplaced. Two decades after the elimination of smallpox, no other disease or disease-causing organism has been similarly banished. Indeed, new and previously-unsuspected diseases—AIDS being probably the most well-known and feared—are emerging with remarkable frequency; and some old, and seemingly defeated, enemies are displaying resistance to everything that medical science has thrown at them so far, and coming back for more.
First the good news—and there is plenty. There have been no cases of polio reported in the Americas since 1991, and its eradication is expected by 2000. Diphtheria cases in the U.S. have been reduced from an estimated 15,000 annually during World War II to only about three a year today; and the number of annual measles cases in the country has plummeted from about 760,000 in 1958 to under 1,000 this decade. But, for much of humanity, infectious and parasitic diseases, or IPDs, remain a deadly threat.
More than 17 million people died from IPDs in 1995, meaning that such diseases accounted for more than a quarter of all human deaths. They are the leading cause of mortality in the Middle East and Asia (outside China) and are responsible for more than half of all deaths in sub-Saharan Africa.
Respiratory diseases such as pneumonia kill more than four million people a year globally and are the leading cause of death among children under five. And despite repeated efforts to stamp it out, tuberculosis refuses to go away, remaining the number one cause of adult death from infectious disease.
In addition to the resilience of long-standing enemies, there is the added complication of “new” and emerging diseases. HIV, Ebola, hantavirus, Hepatitis-C, the Legionella bacterium and the Cyclospora parasite are just some of the almost 30 “new” disease-causing microbes and infectious diseases recognized since 1973, according to the World Health Organization.
The impact of the HIV pandemic in some areas has been devastating, according to a 1997 report by the Population Reference Bureau. In Zimbabwe, where 1994 estimates reckoned 900,000 people to be infected with the virus, some scientists have projected that AIDS may reduce the country’s life expectancy at birth from its 1997 level of 51 years to just 33 years by 2010.
Without HIV/AIDS, Zimbabwe’s life expectancy by 2010 would be expected to be about 70. Why this persistence and resilience? “I think the biggest factor involved in the emergence and spread of new diseases is transportation,” says Dr. S. Jay Olshansky at the Department of Medicine, University of Chicago. “With greater mobility among and between populations, and with global trade resulting in much greater movement across borders, we can now transport disease-carrying organisms across the globe in a matter of hours. At the same time, human populations are also moving into environments where they hadn’t been before, and altering those environments, opening themselves up to organisms they haven’t encountered before.”
Global warming could be another factor. According to Dr. Paul Epstein, associate director of the Center for Health and the Global Environment at Harvard Medical School, “Climate restricts the range at which tropical diseases occur, while weather affects the timing and intensity of outbreaks. We’re seeing an increase in extreme weather events, which increases the incidence of disease. The costs of business as usual are mounting, for the insurance industry, as well as for our health.”
The impact of such environmental change on the spread of diseases is well-illustrated by the case of malaria. The incidences of malaria in Africa, Southeast Asia and South and Central America are generally associated with the clearing of forests, according to a report by the World Resources Institute. For example, the mosquito Anopheles dirus breeds in sunlit pools along the partially-cleared forest margin, where it is responsible for high numbers of malaria cases in South and Southeast Asia. In the Amazon, a combination of rapid population growth, migration and displacement of laborers and indigenous people, environmental destruction and development have created ideal conditions for the growth and spread of malaria, to the extent that reported cases of the disease in the region have risen from 52,469 in 1970 to 577,520 in 1989.
As for the resistance of old diseases, much of the problem there, says Olshansky, “is our own fault. We’ve introduced antibiotics, forcing microorganisms to evolve even faster than they already do. We create new defenses, and they become resistant.” He recommends that most of us can be a lot more judicious in taking antibiotics: “The vast majority of any illnesses from microbes are going to get better on their own. There are times when antibiotics are clearly not necessary.”
It’s a view endorsed by Dr. Mary Wilson of the Harvard School of Public Health, who notes that “physicians are often under tremendous pressure from patients to prescribe antibiotics for problems—such as earaches and many respiratory illnesses—where they plainly won’t have any effect.” However, she adds, even if we all took antibiotics far more sparingly, the problem remains of the “enormous amount of antibacterial agents in the environment. Something like 40 percent by weight of all antibiotics goes into cattle feed. Chopping boards have antimicrobial agents, and so do household soaps. Antibiotics are so over-used that we are in danger of losing many of them.”
A Role for Immunization
Conversely, however, immunizations against viral diseases—sometimes criticized and shunned because of their potential side-effects, particularly on the very young—are, almost all experts agree, essential tools in the fight against disease. The World Health Organization argues that, without vaccination, the risk of death from contracting measles is, in some less developed countries, as high as one in five; the odds of developing encephalitis or a severe allergic reaction, by contrast, are just one in a million. “If everyone was adequately vaccinated around the world,” argues Olshansky, “then most of these diseases would effectively be wiped out.”
Wilson agrees with the importance of widespread immunization, but is less sanguine about its cure-all potential. “Vaccines have been extraordinarily successful,” she says, “but we can’t rely on them. Microbes change, and can get around each and every vaccine. There are lots of other ways to protect people—sanitation, screens on windows to protect them from insects, paying attention to land-use and land-conversion issues. It takes more than medical science to address these issues;
we need a multi-disciplinary approach.”
Even then, the lesson is to be prepared for the unexpected. Many experts now acknowledge that the Hong Kong flu outbreak could have, if Chinese authorities had balked at destroying virtually all the province’s chickens, rapidly developed into a global pandemic. Somewhere in the world, another new organism is lurking, ready to strike. Next time, we might not escape so lightly.