A monkey virus that contaminated polio vaccine given to tens of millions of Americans in the 1950s and '60s may be causing rare human cancers. For four decades, government officials have insisted that there is no evidence the simian virus called SV40 is harmful to humans. But in recent years, dozens of scientific studies have found the virus in a steadily increasing number of rare brain, bone and lung-related tumors - the same malignant cancer SV40 causes in lab animals. Even more troubling, the virus has been detected in tumors removed from people never inoculated with the contaminated vaccine, leading some to worry that those infected by the vaccine might be spreading SV40. By the end of 1996, dozens of scientists reported finding SV40 in a variety of bone cancers and a wide range of brain cancers, which had risen 30 percent over the previous 20 years. Then, Italian researchers reported finding SV40 in 45 percent of the seminal fluid samples and 23 percent of the blood samples they had taken from healthy donors. That meant SV40 could have been spreading through sexual activity, from mother to child, or by other means, which could explain how those never inoculated with the contaminated vaccine ... were being infected. Dr. James Goedert, the chief of the NCI's Viral Epidemiology Branch ... acknowledged that research is needed to resolve the question of whether SV40 is prevalent in the human population and, if so, how it might be spreading. But Goedert said he has no plans for such studies.