Environmental health is concerned with the medical effects of chemicals, pathogenic (disease-causing) organisms, or physical factors in our environment. Because our environment affects nearly every aspect of our lives in some way or other, environmental health is related to virtually every branch of medical science. The special focus of this discipline, however, tends to be health effects of polluted air and water, contaminated food, and toxic or hazardous materials in our environment. Concerns about these issues makes environmental health one of the most compelling reasons to be interested in environmental science.

For a majority of humans, the most immediate environmental health threat has always been pathogenic organisms. Improved sanitation, nutrition, and modern medicine in the industrialized countries have reduced or eliminated many of the communicable diseases that once threatened us. But for people in the less developed countries where nearly 80% of the world's population lives, bacteria...