Several SV40 researchers have criticized Strickler's 1996 study and the more recent British one, saying that they treated specimens in a manner that would not result in the efficient extraction of SV40 DNA. Bharat Jasani, the director of the molecular diagnostic unit at the University of Wales, in Cardiff, has found SV40 in British mesothelioma samples. He recently wrote a lengthy critique of the two studies that has not yet been published. In this critique Jasani concludes that the negative results "are explainable by the paucity of the diagnostic biopsy material used and/or insufficient sensitivity of the overall PCR methodology used." Jasani says that Strickler's PCR technique would have missed low levels of SV40.

Federal health officials are understandably concerned that any link between SV40 and human cancers could frighten people away from the polio vaccine and vaccination in general. They stress that before SV40 in the polio vaccine can be linked definitively to cancer, the proposition must clear important scientific hurdles. Carbone and others must prove that the SV40 they have found is not a laboratory contaminant. They must demonstrate that SV40 is responsible for the cellular damage that leads to cancer and is not just a benign "passenger" in human tumors. And they must show that it was introduced into human beings through the polio vaccine.

In assessing the research to date, Strickler is perplexed that the virus has been found in so many kinds of tumors. In addition to the confirmed research reporting the virus in more than a half dozen kinds of brain tumors and a similar number of bone tumors, researchers in new, isolated studies have reported finding the virus in Wilms tumors, which afflict the kidney, and adenosarcomas, rare cancers of the uterus. "It's not likely that a single virus causes ten thousand different diseases," Strickler says. "That's not how it works."

These anomalies have fueled Strickler's suspicion that many of the SV40 findings in human tumors may really be false positives resulting from laboratory contamination. He points out that SV40 is used for cancer research in so many laboratories around the world that almost any lab involved with tumor assays could conceivably harbor it. "Is it possible that SV40 is in human tumors and that SV40 is at some level circulating in the human population?" Strickler asks. "Could it be true? I can't exclude the possibility, but the studies to demonstrate it haven't really been done, and the data in our hands have been negative." Strickler's former boss, James Goedert, the chief of the NCI's Viral Epidemiology Branch, agrees. Although he says he has an open mind about SV40, he believes that contamination may lie behind the findings of Carbone, Butel, and others.

In 1997, largely in response to Strickler's study, the International Mesothelioma Interest Group set out to determine once and for all if the virus was present in human mesothelioma samples. The organization asked an internationally known molecular geneticist, Joseph R. Testa, the director of the Human Genetics Program at the Fox Chase Cancer Center, in Philadelphia, to oversee a study. Testa, who specializes in mesothelioma research, confesses that initially he doubted the idea that SV40 could be found in human mesotheliomas, because he believed it was well established that asbestos was the cause of the disease. "I'm a very careful person," Testa says. "I had a fair amount of skepticism about it." But the results of the investigation he led changed his mind. Four laboratories participated in the tightly controlled study, including Carbone's. All four found SV40 in at least nine out of the twelve mesothelioma samples they tested. Each laboratory's control samples tested negative, suggesting that the positive SV40 samples were not the result of laboratory contamination. The results were published in the journal Cancer Research in 1998.