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Infectious Diseases

Author(s): Christine Herrmann, PhD

Emerging Infectious Diseases

Although we have developed the technology to control many infectious diseases, new diseases continue to arise and spread. Emerging infectious diseases can be caused by both “new” and “old” pathogens. Diseases that recently appeared in a population, or whose range of hosts or geographic locations are rapidly expanding or threatening to expand in the near future are referred to as “emerging diseases.” Diseases whose incidence had significantly declined in the past, but have again reappeared are known as “re-emerging diseases.” Examples of emerging diseases are HIV, influenza, SARS, and Ebola. Tuberculosis is a disease that is re-emerging. “Old” pathogens are considered emerging if they are spreading to new species or new geographic locations. 

Diseases sometimes emerge due to natural processes, such as the evolution of the infectious agent over time, but many emerging diseases result from human activities. These include population growth, migration from rural areas to cities, international air travel, blood transfusions, poverty, wars, and destructive ecological changes caused by economic development and land use.

Many emerging diseases arise when infectious agents in animals are passed to humans (referred to as zoonoses). This is occurring more and more as the human population grows and spreads into new geographical regions, resulting in increased contact with wild animal. It is not difficult to imagine a scenario in which a hunter in a remote area of Africa becomes infected with a “new” infectious agent and then travels to a village, where he infects other locals, one of whom boards a plane and infects his fellow passengers, who then travel to other parts of the world before any infection is detected. In this way, a disease could emerge quite rapidly. 

For an emerging disease to become established, at least two events must occur: (1) the infectious agent must be introduced into a vulnerable population; and (2) the agent must have the ability to spread readily from person to person, cause disease, and sustain itself within the population. Both of these events have occurred with HIV. To date, only the first has happened with avian flu.

Even diseases previously thought to have been under control can sometimes make a comeback. This can occur when an agent becomes resistant to the drugs, such as antibiotics, used to treat the disease. An infectious agent can mutate so that these drugs are no longer useful in combating the disease. Drug resistance is on the rise, in large part due to overuse and misuse of antibiotics and other drugs. Tuberculosis, for example, is becoming highly antibiotic-resistant. Malaria, too is becoming increasingly drug-resistant.

A frightening possibility is the emergence of an infectious disease due to its deliberate introduction into human or agricultural populations for terrorist purposes. Since the terrorist attacks of September 11, 2001, and the subsequent mailing of anthrax-laced letters, this threat is being taken very seriously. Agents considered most dangerous include those which cause anthrax, botulism, plague, and smallpox. Other potential agents classified as risks, albeit lower level ones, include West Nile virus, Salmonella, SARS, influenza, yellow fever, and drug-resistant tuberculosis.