A growing number of medical researchers fear that 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.

The researchers noted that the children were too young to have received the contaminated vaccine. But somehow the virus had infected them and embedded itself in their tumors.

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, such as the Boston children, were being infected.

But officials did not recall contaminated Salk vaccine - more than a year's supply - still in the hands of the nation's doctors.

And they did not notify the public of the contamination and SV40's carcinogenic effect on newborn hamsters.

It was possible that this second strain of SV40 had been evading manufacturers' screening procedures for years - and infecting vaccine recipients after 1962.