Scientists have discovered traces of a monkey virus called SV40 that contaminated the polio vaccine in the 1950s in a common form of cancer called non-Hodgkin's lymphoma. Over the last 30 years, cases of non-Hodgkin's lymphoma have risen by 100 percent, and the disease is currently ranked fourth or fifth among cancer deaths in the United States. The Salk polio vaccine, which was administered to people between 1955 and 1963, was developed using kidney tissue from rhesus monkeys. At the time, the manufacturing process was thought to be safe, but in 1960, it was discovered that large batches of the vaccine had been contaminated with SV40. An estimated 30 million Americans were possibly exposed to the virus, and in the 1990s new DNA detection techniques started to find evidence of SV40 in human tumors, including rare lung, bone, and brain-related cancers called mesotheliomas. Other research has revealed that the simian virus is also present in adults and children born after the contaminated vaccine was taken off the market, deepening the mystery of how the virus is being passed on. Two studies that found a connection between SV40 and non-Hodgkin's disease were published Friday in the British medical journal The Lancet. One study discovered that SV40 may be involved in a wide variety of human cancers, while Dr. Adi Gazdar of the University of Texas, who led the second study, said that recent discoveries about SV40 could lead to more effective cancer treatment. [Polio vaccines in use since the early 1960s have remained free of SV40.]