For a few exhilarating decades in the middle of the twentieth century, it seemed the world might have a reprieve from some major infectious diseases. After coordinated worldwide efforts had virtually eliminated smallpox and made major inroads against other infectious diseases such as influenza, tuberculosis, and polio, some public health officials thought we had entered a new era in which infectious diseases would no longer be among the planet's worst killers. By the 1980s, though, those hopes were dashed, due in large part to the burgeoning AIDS epidemic.
Beginning around the same time, dozens of other infectious diseases--including Lyme disease, hantavirus pulmonary syndrome, West Nile virus, and Ebola hemorrhagic fever--were either newly recognized, spread to new locations, or became increasingly deadly. In 2003 alone, SARS, avian influenza, and monkeypox were among the infectious news makers. Along with the "new" diseases, long-time threats such as malaria surged back, now killing about 3 million people each year and infecting at least 300 million more, according to the 2002 U.S. National Institute of Allergy and Infectious Diseases (NIAID) publication Microbes: In Sickness and in Health.
More than a dozen major factors have been pegged over the past 10-15 years as existing or potential contributors to infectious disease outbreaks. Microbes continue to evolve rapidly, and human susceptibility to infection is increasing due to malnutrition, weakening of immune systems through cancer treatments or chronic diseases, and aging of the population in some countries. Extensive poverty and population growth have spawned impacts such as poor basic public health infrastructure in megacities. Wars result in dislocation of peoples, destruction of public health systems, malnutrition, and introduction of new microbes by soldiers returning home; bioterrorism raises the possibility of infectious agents being released upon the population at large. Changing medical technology allows exchanges of microbes through blood transfusions and organ and tissue transplants, some of which come from animals. Increased international travel and exchange of goods--including animals, plants, and foods that can carry pathogens--both spread diseases rapidly. Many nations have seen reduced funding for safe water and sewer systems, vaccines, research, surveillance, prevention, and response, due in part to complacency.
As if those driving forces weren't enough to contend with, many environmental factors are playing major roles, including climate change, deforestation, global dust transport, and numerous agricultural practices. The links between these factors and millions of potential human deaths have often been overlooked or discounted as pollution and other more obvious concerns have typically drawn the limelight.
Emerging and Surging
The complex relationship between people and microbes has been evolving for tens of thousands of years. Many microbes are essential to our health. But a few bacteria and many viruses, fungi, and protozoa can cause infectious diseases and play a role in cancers, coronary heart disease, diabetes, multiple sclerosis, autism, and chronic lung diseases.
With the accelerated development of vaccines and antibiotics in the past century, along with major regional and worldwide drives to use these new tools, some of the worst offenders have been beaten back. Smallpox, for example, which some researchers say at one time had killed more people than all other infectious diseases combined, was virtually eliminated in 1977 thanks to a global vaccination effort. Along with the success against smallpox came strides against polio, malaria, measles, guinea worm disease, and other nemeses.
More than 1,400 species of infectious microbes are known to be human pathogens, and of these, 175 fall into the "emerging" category, wrote Louise H. Taylor and colleagues from the U.K. …