Forty-one years ago, millions of peasants, soldiers and students across rural China lined the banks of dried-up rivers and lakes before the start of the rainy season, and turned the soil by hand in an effort to bury the snails that carry the parasite that causes schistosomiasis.

Under the orders of Communist Party Chairman Mao Zedong, banners flew with slogans declaring "Our strength is boundless, our enthusiasm redder than fire," and "Empty the rivers to wipe out the snails, resolutely fight the big belly disease." An article in the Chinese Medical Journal of August 1958 hailed "the people's boundless energy," claimed widespread success and proclaimed that Wuhu county was "a snailless county."

Four decades later, studies here in the Wuhu area and across China are exploding another myth of the Mao era. The snails -- and schistosomiasis -- are alive and well and remain a gnawing problem in China. Other sorts of worms and parasites are even more abundant. Based on a 1990 nationwide survey of a million people, epidemiologists estimate that about 700 million Chinese -- a staggering 62.6 percent of the population -- harbor one or more types of parasites.

Most of them live in the Yangtze River basin where the Three Gorges Dam will soon make a huge and unpredictable impact, probably sharply increasing the rates of certain diseases, such as schistosomiasis, in some areas and possibly lowering infection rates in other areas.

Infection rates for parasites have probably dropped in recent years, as China has grown more urban and peasants begin using more chemical fertilizers rather than human and animal excrement. But the "unholy trinity" of the parasite world -- Ascaris lumbricoides (roundworm), Trichuris trichiura (whipworm) and Ancylostoma duodenale (hookworm) -- still plague much of rural China and are stunting children's growth, damaging their mental abilities and making them lethargic and anemic. The number of Chinese with hookworm alone is nearly 200 million.

"Though there is no hookworm in cities, go a few miles outside Chinese cities and you go back in time," said Peter Hotez, a Yale University pediatric epidemiologist and one of the world's few hookworm experts. "The great cities in China are very modern, as modern in some ways as Los Angeles. But two hours away, nothing has changed in a very long time, and people are still using human feces as fertilizer."

Last month, Hotez journeyed to one of the villages south of Wuhu in search of worms. He traveled two hours by car, then took a pleasant walk through fields bursting with yellow rapeseed plants. A rusty metal boat carried him across a tributary of the Yangtze River, then he took another short walk to Zhongzhou village, where the muddy streets are shared by pigs, cows and people, most of whom had never seen a Caucasian person, much less one dressed in corduroy pants, a blue oxford shirt, a tie and a blue blazer.

A survey here in March indicated that 36 percent of the 2,567 people who live in this village in the southern corner of China's Anhui province have hookworms.

"These are the exact same conditions we use at the lab to cultivate hookworm," Hotez said as he passed the fecund fields, which will soon be full of tobacco and cotton.

Hookworm, which Hotez estimates infects up to a billion people worldwide, was once common in the American South. The fight against hookworm was one of the first aims of what later became the Rockefeller Foundation. The worm vanished in the United States early this century as sanitation improved.

Magnified, the worms look menacing, with tapered bodies and sharp teeth. The worms, which can measure anywhere from a half-inch long to four inches, suck blood from their human hosts, causing anemia, stunting their growth and damaging their intellectual capacity. Although one worm does little harm, a person with hookworm could have hundreds or thousands of them and lose as much as a cup of blood a day, causing severe loss of iron and protein.

Although treatable, hookworm tends to reinfect people who continue the same habits that caused them to contract schistosomiasis in the first place. The worms like damp, cool places, and fields of rapeseed, cotton and tobacco are ideal. Eggs deposited in the soil develop into larvae, which are swallowed or attach to passing humans or animals and penetrate the skin.

Once in the bloodstream, the larvae pass through the heart and into the lungs and airways, where they are coughed up and swallowed. When they reach the small intestine, the larvae mature into adult worms and attach themselves to the intestinal wall. Adult worms live an average of four to five years.

To combat the disease, Hotez is trying to create a vaccine. To do that, he has to figure out why some people get hookworm while others in the same village do not, and why some people get mild cases and others are afflicted more severely. If he can identify what makes people less susceptible, he can isolate it and put it into a vaccine.

Hotez said he has four or five candidates for a hookworm antigen that have produced "promising" responses in mice. He hopes to raise funds to try to develop a vaccine. But many American foundations have moved away from funding basic scientific research in favor of health-care policy studies. Even though one in five people on the planet has hookworm, big drug companies do not want to fund hookworm research because the people who have hookworm -- the poor of China and India -- can't afford to buy a drug even if someone like Hotez can develop one.

The Institute of Parasitology in Wuhu is like a museum of intestinal worms. Fingernail-size hookworms are preserved in small vials. Giant lung flukes that cause pulmonary disease are preserved in large jars.

Facilities for examining and testing new samples, however, are rudimentary. China has paid scant attention to the problem of parasitic diseases as it focuses on industrial development. Although the Anhui parasitic disease control unit has 5,000 employees, the government has devoted few financial resources to them. Training for the most part is poor, and researchers have at best a rudimentary idea of how to approach the problem.

At the county branch of Anhui's parasite-control bureaucracy, Hotez examined a map of Zhongzhou village and urged health workers to identify which households have the most cases of hookworm. He wants them to return to the village and take blood samples from children, who were left out of the initial survey. When he got to the village, he understood one reason why. No one had needles the right size for children.

The village party secretary greeted him, but the village doctor could not be found. Hotez examined a tiny child who, according to the mother, is 8 years old. The embarrassed village leader said the child is 5.

Hotez will have some needles sent from the United States to researchers in Shanghai, who will have to return here. They will then bring samples back to Shanghai, freeze them and extract DNA from them.

One thing that could change the parasitic map of China is the construction of the giant Three Gorges Dam across the Yangtze River. At the moment, the natural gorges, forming a relatively steep and rocky section of the mighty waterway, divide China into different parasitic regions, effectively quarantining areas above the gorges from those below. "The Three Gorges act like a meat grinder. Nothing survives going down the Three Gorges," said George Davis of the Academy of Natural Sciences in Philadelphia. Davis has examined parasites that inhabit the stagnant waters on the flood plains around Boyang Lake. "The parasites above the gorges have their own genetics, and those below have theirs."

But after the dam is built, parasites will be able to travel up and down the river through the new ship channel. Above the dam, a huge reservoir will be created, a perfect habitat for the snails that cause schistosomiasis. Some areas with better flood control might be better off, however.

Another factor that will alter the lives of worms will be the temperature change caused by the dam. The giant reservoir formed when the dam is completed will moderate temperature fluctuations in Sichuan province, and promote further the survival of parasites that don't like extreme heat or cold.

"No one can definitely say what will happen," said Feng Zheng, a doctor at the Chinese Academy of Preventive Medicine in Shanghai. CAPTION: Peril of a Parasite

About 1 billion people worldwide are infected with hookworms that are particularly widespread in developing countries. The worms, which attach themselves to the intestinal walls sucking blood, can stunt children's growth, cause lethargy and anemia. In China alone, 200 million people are infected. 1. Hookworm larvae enter the body through the skin, often through the feet of people walking barefoot. 2. The larvae travel in the bloodstream to the lungs and airways. 3. Larvae are coughed up and reach the stomach. 4. From the stomach, the larvae reach the intestines, where they grow into adult worms, about the size of a fingernail. They attach themselves to the intestinal walls, and suck blood. One person can have thousands of worms, which together can suck as much as a cup of blood a day. SOURCE: Staff reports CAPTION: A farmer stands by a river that provides irrigation in Zhongzhou but also is a home to the hookworm. CAPTION: A farmer carries compost, a staple of farming in China and a breeding place for the hookworm. CAPTION: Hookworm researcher Peter Hotez gets together with children in Zhongzhou.