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A World in a Drop

Any drop of untreated water, from a lake, a river, or the ocean, is a world in miniature. The drop can contain thousands of tiny organisms, such as algae, protozoans, bacteria, and viruses. Most of these microbes are harmless to other animals, including humans--fewer than 1 percent of bacteria cause disease, for example. Yet those few harmful types of microbes, consumed accidentally in a drink of water, can be dangerous.

By the Numbers

Worldwide, up to five million deaths a year are due to water-related diseases.

Spinning Wheel

The microorganisms called rotifers were so named because of the two rotating wheel-like lobes that sweep food into the animal's mouth and propel it along. Many rotifers are water purifiers, feeding on algae and suspended particles.

The Wave

Single-celled microorganisms called Oscillatoria owe their name to the way they move, gliding with a waving motion--or oscillation--through the water. Oscillatoria can be found in fresh water, ocean water, and even hot springs.

When someone drinks water contaminated with Giardia they swallow this cyst, one of two forms Giardia takes during its life cycle. The cyst form has a hard protective covering, but once inside a digestive tract--animal or human--it changes into a form that moves with the aid of tiny arms, or flagella. In this form it attaches to the intestinal wall, possibly interfering with the intestine's ability to absorb fluid and resulting in what may be severe diarrhea.

Deadly Cyst

A one-celled organism, called Cryptosporidium, forms a coating called a cyst during one phase of its life cycle. This cyst resists chemicals such as chlorine and allows the microbe to survive for a long time outside the body. A 1993 outbreak of Cryptosporidiosis in the U.S. city of Milwaukee, Wisconsin, caused by a compromised water treatment plant, killed more than 100 people.

A Bit of Both

The microbe, Euglena, is a bit like a plant and a bit like an animal. It moves around by waving a tail-like structure called a flagellum, but it contains a substance called chlorophyll that helps it convert the Sun's energy directly to food, the way plants do. Euglenas are often found in sewage treatment systems, or in other nutrient-rich freshwater.

Small Hunter

An oval-shaped creature, Didinium, lives mostly in fresh or slightly salty water and favors one kind of prey, another protozoan called a paramecium. It paralyzes its victim with poison darts called trichocysts (TRICK-oh-sists), then swallows it whole.


© Marc Steinmetz/Aurora Photos

Light Sanitation Water

The Face of Death

The course of the waterborne disease called cholera is frightening in its speed: intense thirst, acute cramps, and diarrhea, then death from dehydration, often by the third day. It is also preventable. With water treatment and sanitation--that is, proper waste disposal--cholera has been virtually eliminated in wealthier nations. Yet this killer still stalks the developing world, where more than 2.6 billion people lack basic sanitation. Many cholera victims are children.

Poisoned Well

A London doctor, John Snow, first made the link between cholera and water in 1854. Snow pinpointed the houses of cholera victims on a city map and realized that most got their drinking water from the same public well, likely contaminated by an overflowing cesspool.

By the Numbers

The microbes that cause cholera are tiny. A cup of water could easily contain millions of them and be perfectly clear.


Courtesy Dr. Rita Colwell and Anwarul Huq / NSF


Tit for Tat

The waterborne bacterium that causes cholera, Vibrio cholerae, has evolved a partnership with tiny aquatic creatures called copepods. The bacteria hitchhike on the swimming copepods as a way to move through the water, clustering around the mouth and egg casing of females. In "payment," V. cholerae assist in copepod reproduction by secreting a substance that helps rupture the egg casing.



Water Temperature

A Permanent Presence

The oceans are a permanent reservoir of the cholera bacterium, and water temperature is critical to its life cycle. When the water is cold, Vibrio cholerae are dormant and nearly undetectable. When the water warms, they can sometimes multiply. Though the disease does not occur in all warm waters, scientists can now predict some cholera outbreaks by measuring water temperature. Warm water is yellow or red in the satellite image at right.

A Simple Defense

In some places drinking water is treated with chemicals such as chlorine, greatly reducing mortality from waterborne diseases like cholera. In the absence of such treatment, resourceful women in Bangladesh have been applying the simplest of cholera defenses. By filtering drinking water through a sari folded eight times, they have cut cholera rates in half. Saris are then rinsed in filtered water and dried in the sun.

Filter as You Go

Inexpensive personal filters are promising tools for combating waterborne disease. They are used when there is no convenient source of safe water. Some use a textile filter to screen out tiny particles; a disinfectant to eliminate the bacteria responsible for diseases such as cholera, dysentery, and typhoid, and activated charcoal to improve the taste. Filters on your taps at home can process up to several thousands of liters and may improve the taste of your water.

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