A simian virus known as SV40 has been associated with a number of rare human cancers. This same virus contaminated the polio vaccine administered to 98 million Americans from 1955 to 1963. Federal health officials see little reason for concern. A growing cadre of medical researchers disagree. A breakthrough in the war against polio had come in the early 1950s, when Jonas Salk took advantage of a new discovery: monkey kidneys could be used to culture the abundant quantities of polio virus necessary to mass-produce a vaccine. But there were problems with the monkey kidneys. In 1960 Bernice Eddy, a government researcher, discovered that when she injected hamsters with the kidney mixture on which the vaccine was cultured, they developed tumors. Eddy's superiors tried to keep the discovery quiet, but Eddy presented her data at a cancer conference in New York. She was eventually demoted, and lost her laboratory. The cancer-causing virus was soon isolated by other scientists and dubbed SV40, because it was the fortieth simian virus discovered. Alarm spread through the scientific community as researchers realized that nearly every dose of the vaccine had been contaminated. In 1961 federal health officials ordered vaccine manufacturers to screen for the virus and eliminate it from the vaccine. Worried about creating a panic, they kept the discovery of SV40 under wraps and never recalled existing stocks. For two more years millions of additional people were needlessly exposed -- bringing the total to 98 million Americans from 1955 to 1963.