About half the world's reported cases of polio, a crippling disease virtually wiped out in Western countries, occur in India. Each year, diarrhea kills 500,000 Indian children. A jaundice epidemic strikes a small district of India's Rajasthan state as regularly as the annual monsoon.

Those deadly diseases and others that afflict India can be traced to the same source: drinking water contaminated by human waste. Infected water causes an estimated 80 percent of disease in India, according to the World Health Organization (WHO), making poor sanitation and inadequate sewage disposal the nation's biggest public health problems.

"Waterborne diseases in India are very, very common. Every year, there's bound to be a few epidemics of viral gastroenteritis, typhoid, cholera," said P.C. Bhatnagar, a community health worker for the Voluntary Health Association of India.

Fewer than 30 percent of India's 950 million people have bathrooms in their homes or easy access to public toilets. The rest routinely relieve themselves in the open -- along roadsides, on farmland or in municipal parks.

No more than 250 of the country's 4,000 cities and towns have sewer systems, and many of those systems do not have treatment plants. The bulk of municipal sewage -- even from such major cities as Bombay and Calcutta -- flows untreated into rivers, lakes or the sea.

Other developing countries in Asia and Africa have similar health problems because, like India, they cannot afford the heavy public expenditure needed to build sewer systems. But diseases related to unsanitary conditions have been more prevalent in India because of its huge population, according to analysts.

"If you look at waterborne diseases, the largest burden of disease is here," said John Pospisilik, an environmental engineer at the WHO office here in New Delhi, the capital.

For two years, India has participated in a WHO campaign to eradicate polio by 2000 through universal vaccination. The government also has joined private organizations in adding bathrooms to rural homes, but at the current rate of construction and population growth it would take 200 years for every Indian to have access to a toilet.

Besides overpopulation, religious beliefs of Hindus, who account for 80 percent of the population, have compounded India's sanitation problems. The cities of an early civilization in the Indus River valley had sophisticated sewer systems and among the oldest known toilets -- brick models that date back 4,500 years. But the development of Hinduism and its caste system in later centuries changed attitudes and practices concerning the disposal of human waste.

Bindeshwar Pathak, whose private organization, Sulabh International, has built 700,000 toilets in 25 years, said an ancient Hindu text gave "firm religious sanction" to unsanitary behavior by forbidding defecation near dwellings. "It's very difficult to bring it into the homes," Pathak said. "It's a cultural problem in India."

In rural areas, where more than 70 percent of Indians live, fewer than 10 percent of homes have toilets. Government officials and aid workers say they have experienced difficulty persuading uneducated villagers to abandon ancient customs and use an enclosed bathroom. In the northern state of Rajasthan, some villagers have converted outdoor latrines into storage areas and resisted construction of indoor bathrooms, fearing they would make their homes smell bad.

Officials involved in programs to install household toilets in villages often neglect to conduct public education campaigns to establish the connection between sanitation and health.

In rural schools, instruction in such basic hygiene as hand-washing is limited and cannot be reinforced in school buildings that lack running water and lavatories.

In cities, hundreds or even thousands of people may use the same public toilet each day, causing them to reek if not cleaned frequently.

Such conditions help explain why one day recently two men could be seen urinating on the outside walls of public toilets on opposite corners of a major intersection here.

Almost two-thirds of urban dwellings have bathrooms, according to a 1991 census, and middle-class city dwellers generally practice good hygiene. Still, public health risks are greater in cities than in rural areas because of cramped conditions in slums where the poor -- nearly half of Bombay's 13 million residents, for example -- live in shanties without toilets or sewer connections. Under a $300 million project funded by the World Bank, Bombay plans to treat the 60 percent of the city's sewage now discharged raw into the Arabian Sea. The seven-year project, which also will build public toilets for 1 million slum dwellers, is the latest phase of water and sanitation improvements that India's biggest city began in the 1970s.

Traditionally, efforts to improve sanitation in India have not had public health as their main motivation. Instead, it has been liberation of low-caste workers formerly called "untouchables" from the degrading occupation of cleaning waterless toilets and carrying away woven baskets of human waste on their heads. Hinduism teaches that contact with human feces defiles members of the upper castes.

These religious beliefs frustrated a crusade launched a century ago by Mohandas K. Gandhi, who led India to independence in 1947. Gandhi defied tradition by cleaning his own toilet and urging other members of upper castes to do the same. He also criticized Indians for casually relieving themselves in the open.

"It filled me with agony to see people performing natural functions on the thoroughfares and river banks, when they could easily have gone a little farther away from public haunts," Gandhi wrote in his autobiography of a 1915 visit to the Ganges River.

Little has changed in the intervening decades. The leaders of independent India concentrated the nation's limited resources on industry and scientific expertise, largely ignoring such basic health problems as sanitation and hygiene. Until a decade ago, the government was constructing houses for the poor that did not have bathrooms.

As in Gandhi's day, villagers still rise early in the morning and walk to the fields, not returning home until nightfall. The human waste they leave behind acts as an organic fertilizer, but a potentially unhealthful one that can infect farm workers and contaminate produce.

In Viratnagar, a village in Rajasthan, the government last year built outdoor toilets for 800 houses, but the village's elected leader recently estimated that only 100 are in use.

The program -- financed by the Rural Development Ministry, which declined to make a top official available for comment -- had other failings as well. The toilets installed outside each home were not enclosed by walls and were placed near the front door. Each villager, who paid $7, about one-tenth of the cost, was expected to erect an enclosure, but many could not afford the additional expense.

Months later, most of the toilets were unused, filled with mud or covered with bricks and earthen jars.

"I did not urinate even once in it," said Bahru Lal Kumhar, a potter. "Where are the walls? . . . The fields are open -- nobody comes there. But this is in front of my house. Everybody passes by. My family would see."

In another part of Rajasthan, a private aid group, the Indian Institute of Health Management and Research, has achieved more success with a decade-old program combining health education and the installation of household toilets. The number of small children dying from diarrhea has decreased in the area, aid workers said.

When Bhika Ram Sharma, a sanitation worker who lives in the same village, built an enclosed toilet in his residential compound a decade ago, he became the object of scorn because he is a Brahman, the highest caste.

"People ridiculed me. They laughed at me and said, That Brahman boy has built a latrine inside his house,' " he recalled.

But after years of continuous health education, a desire to keep up with the Sharmas began to take hold in the village. Now, the group has a waiting list for the installation of toilets under a program that requires each village only to make enough bricks to line the pit and to erect the walls.

"It takes time," Sharma said. "We can't change the culture entirely in just five years, a culture that has been going on for centuries." Special correspondent Rama Lakshmi contributed to this report. CAPTION: ASIAN HEALTH PROBLEM Almost 2 billion people in the developing world have no access to sanitation. This situation causes huge public health problems, and officials say that 80 percent of all diseases in those regions are caused by poor sanitation and contaminated water. Access to sanitation services in major South Asian countries These include toilet facilities in the home or at a convenient distance; countries apply varying definitions and most include simple latrines.

URBAN

RURAL COUNTRY

POPULATION 1980 1991 1980 1991 Bangladesh 120 million n.a. 50% n.a. 7% India

950 million 27 53

1 2 Indonesia

201 million 29 64

21 36 Pakistan

134 million 42 55

2 10 Philippines 72 million 81 78

67 62 Sri Lanka

18 million 80 73

63 56 A large number of people in Asia, Africa and Latin America have access to some kind of simple latrine. Shown here are the population segments at either extreme.

No access to

Access to

any kind of

municipal

toilet facility sewer system Asia population:

1.4 billion

300 million 3.5 billion Africa:

380 million

30 million 730 million Latin America:

160 million

190 million 490 million SOURCE: U.N. Center for Human Settlements (HABITAT) CAPTION: A man leaves a public toilet in a Bombay slum. In cities, 53 percent of people have easy access to toilets; in rural areas, only about 2 percent have such access.