Water-associated pathogens that threaten human health include, from left to right, Norovirus, 

 the Norwalk virus; Legionella, the bacterium responsible for Legionnaires' disease, a kind 

 of pneumonia; Plasmodium, the protozoan that causes malaria (pictured here inside a red 

 blood cell); Cryptosporidium, a protozoan that causes severe diarrhea and resists treatment 

 with chlorine; Vibrio cholerae, the cholera bacterium; and Giardia lamblia, another protozoan 

 that causes diarrhea. 



prove water quality worldwide; benefits could come from 

 actions as simple as maintaining hillside growth to prevent 

 soil erosion and flooding. But because many watersheds 

 span several states or even countries, most management 

 plans are politically complex. A comprehensive watershed- 

 management plan must incorporate multiple stakeholders' 

 needs and conflicting interests. 



W: 



ater scarcity goes hand in hand with disease. As 

 renewable freshwater becomes a dearer commodity 

 worldwide, waterborne disease agents and other 

 contaminants become harder to control. When deal- 

 ing with diarrheal diseases, for instance, the quantity of 

 available water often matters more than the quality, both 

 to fend off the disease and to foil its spread. Then there's 

 trachoma, a condition that can cause blindness; today it 

 affects 6 million people and is associated with poor personal 

 hygiene, often resulting from a dearth of water. 



Every person, every day, needs at least thirteen gallons 

 of water for drinking, cooking, bathing, and sanitation. 

 In 1990 more than a billion of the world's people used less 

 than that. By contrast, average per-capita water usage in 

 the U.S. now exceeds 150 gallons a day. That discrepancy 

 illustrates how the level of personal use correlates not only 

 with the economic development of a region, but also with 

 the degree of urbanization and with the overall public 

 health in the region. 



All that water filling swimming pools and soaking 

 gardens might seem extraordinarily wasteful, but only 

 8 percent of the planet's freshwater supply goes toward 

 personal, household, and municipal water use. Agriculture 

 accounts for 70 percent, and industry for 22 percent, of 

 current freshwater use. It takes more than fifty gallons 

 of water to produce a single cup of milk. That's modest 

 as virtual water content goes: consider a quarter-pound 

 hamburger (470 gallons) or a cotton T-shirt (520 gal- 



lons). Then consider how many cotton T-shirts are 

 tucked away in your closets. It's no surprise that demand 

 is exceeding supply. 



Daily water needs are exceedingly hard to meet in 

 areas where rapid urbanization is taking place. 

 Antiquated water-supply systems are simply not 

 equipped to provide enough water and sanitation to 

 people living in progressively crowded shantytowns or 

 on the urban fringe. About half the world's people are 

 now city dwellers. This new urban majority puts great 

 stress on infrastructure, increasing the likelihood that 

 illegal connections will be inserted into existing water 

 systems and that, as a result, the piped drinking water 

 will become contaminated. 



Countries undergoing urban population booms often 

 face acute microbial hazards. In countries where per- 

 capita income is low, roughly 200 children under the age 

 of five die every hour from a water-associated microbial 

 infection. Many of the infections derive from the inges- 

 tion of water contaminated with human or animal feces 

 that carry pathogenic bacteria, viruses, protozoa, or hel- 

 minthes. That's the classic, but not the only, pathway for 

 waterborne disease spread. 



Exposure to contaminated water extends beyond the 

 drinking fountain. Many diseases, once introduced into 

 a population, can spread via person-to-person contact, 

 in aerosol droplets, or through food preparation, rather 

 than direct consumption of contaminated water. For ex- 

 ample, malaria-carrying mosquitoes use stagnant water 

 as a breeding ground; Giardia can be acquired during a 

 swim in a local lake; clothing or bedding may carry scabies 

 mites; noroviruses can be transmitted by eating oysters 

 [see photomicrographs on these two pages] . 



Emerging infectious diseases (the ones whose incidence 

 in humans has increased in the past two decades or threat- 



48 



natural history November 2007 



