Modern Factors in the Spread of Disease
On March 30, 2001, the US Centers for Disease Control and Prevention (CDC) in Atlanta was notified of an outbreak of acute respiratory infections among college students who had visited Acapulco, Mexico, for spring break. By April 9, 37 colleges in 18 states and the District of Columbia had reported more than 221 cases of seriously ill college students, all with similar symptoms: fever, cough, shortness of breath, and chest pain. Some students were sick enough to require hospitalization. A rapidly organized investigation revealed that the cause of the illnesses was not influenza, Legionnaires' disease, or any other virus, but rather infection with Histoplasma capsulatum, a fungus found in soil that causes infection when its spores are inhaled. Investigations are ongoing to determine the circumstances or activities that led to so many acute infections. Fortunately, this particular infection is not contagious, unlike many other respiratory illnesses.
New and unusual diseases, outbreaks, and microbial threats seem to have become a regular part of daily life. Newspapers and journals are full of such alarming reports: a new virus in Malaysia that affects pigs and spreads to humans; a new outbreak of the Ebola virus, this time in Uganda; a severe form of influenza in Hong Kong that can spread directly from chickens to humans; outbreaks of multi-drug-resistant tuberculosis in Russian prisons and elsewhere; drug-resistant forms of common maladies, such as malaria, pneumococcal pneumonia, and staphylococcus. Looming above all, HIV/AIDS spreads relentlessly, transforming population structures by killing young, productive adults and causing life expectancies to plummet, especially in areas of Africa.
Today we are seeing the emergence of new diseases, the spread of well-known pathogens into new populations and regions, the resistance of old pathogens and pests to standard therapies, and new interactions between diseases. Some view the interactions between microbes and humans as a battle, with the microbes in pursuit of humans, or at least waiting to attack. The reality is that humans have changed the environment and ecosystem in ways that favor the global dispersion of microbes and their genetic information.
Infectious diseases are one manifestation of a dynamic system that includes the physicochemical environment and biological life. Population size, density, distribution, mobility, and vulnerability, as well as social and political factors, influence the appearance, spread, and burden of infectious diseases. The global situation today favors the appearance, recognition, and spread of infectious diseases in people, plants, and animals. Although this article focuses on infectious diseases in humans, infectious diseases in plants and animals also have major economic impacts and can have both direct and indirect consequences for human health.
The immediate sources of microbes that cause disease in humans are typically another human, an animal or insect vector, soil, or water. The relative importance of these modes of transmission is made evident in a 1996 World Health Organization (WHO) report that analyzed the breakdown of infections causing death on a global scale. Approximately 65 percent of these infections were spread from person to person (e.g., AIDS, influenza, tuberculosis), 22 percent originated from food, water, or soil (e.g., cholera, typhoid fever, Legionnaires' disease, histoplasmosis), 13 percent were transmitted by an insect vector (e.g., malaria, dengue fever), and 0.3 percent came directly from animals (e.g., rabies).
For infections that spread from person to person, local and global movement has never been easier. The world population is 6.1 billion and is expected to grow to 9.3 billion in 50 years. Currently about half of the world's population lives in urban areas, more than ever before. Increasingly, large population centers are concentrated in low-latitude areas that also tend to be underdeveloped. …