In the 40 years since its founding on April 7, 1948, the World Health Organization (WHO) has thrived on daunting challenges. Indeed, the overall goal of this specialized agency of the United Nations is nothing less than "the attainment by all peoples of the highest level of health."

At present, many of WHO's efforts are focused on developing a global strategy to combat AIDS. Dr. Jonathan M. Mann, director of the agency's Special Program on AIDS, constantly stresses the need for international cooperation. "When a vaccine (for AIDS) becomes available," he said last fall, "WHO is going to make sure that it's available to the whole world and not just to the rich."Over the years, WHO has done notable work in checking such dread diseases as polio, leprosy, malaria, cholera and tuberculosis. It has also made studies of medical health and of the standardization of health statistics and drawn up a code to control the marketing of infant formula in Third World countries.

But WHO's greatest triumph to date, most public health officials would agree, has been the eradication of smallpox. In 1967, the agency launched an intensive campaign against the highly contagious disease with the aim of eliminating it within a decade. Smallpox was reported in 42 counties that year and killed some 2 million people. Eight million other victims were left blind or heavily scarred.

The anti-smallpox project was greatly facilitated by such technical advances as freeze-dried vaccine (a type that will not deteriorate in the tropics) and the jet injector, a gun capable of inoculating 1,200 persons per hour.

Though WHO failed to meet its unofficial deadline for wiping out smallpox, it came fairly close to doing so. The last documented case of the disease through natural transmission occurred in October 1977 in Somalia. After a waiting period of more than two years, during which no additional cases came to light, WHO declared total victory over smallpox on May 8, 1980. Eradication was possible because smallpox was a disease passed only from one person to another.

More recently, WHO has reported considerable progress in combating onchocerciasis, "river blindness," a parasitic disease transmitted by black flies that breed in flowing water. Under a program dating from 1974, infested areas of 11 countries in sub-Saharan Africa have been sprayed with larvacides during the rain season in an effort to prevent the flies from reaching maturity.

Today, WHO claims, 90 percent of the program area is safe for resettlement. This should provide a much-needed boost to the economies of the affected countries, for the agricultural lands adjoining West African rivers are among the most fertile and well-watered on the entire continent.

Eradication of river blindness probably is not possible with the weapons currently at WHO's disposal. But the agency expresses confidence that the disease is now sufficiently under control to permit local health authorities to assume responsibility for monitoring it.

Looking ahead, WHO hopes to meet yet another ambitious objective to immunize all the world's children against the major infectious diseases by 1990. The agency estimates that 3.5 million children a year die from ailments preventable by existing vaccines that are safe and effective.

As with smallpox, WHO may well miss its target date. But given the agency's past performance, few observers would dismiss its chances of succeeding evenutally.