"For decades, the polio vaccine had been made in cells taken from monkey kidneys, some of which – it was later discovered – were infected with a virus, simian virus 40 (SV40). Though today vaccines are extensively filtered, and don’t contain any material from the cells they’re grown in, between 1955 and 1963, it’s been estimated that up to 30 million people were infected in the United States alone. The contamination is thought to have occurred because the cells were usually grown fresh from monkeys – as opposed to from a stock of laboratory cells – and SV40 is a common infection in the most widely used species, the rhesus macaque. Over the ensuing years, frozen vials of the cells were flown to hundreds of laboratories across the world. Whether the introduction of the virus had any medical consequences is still under question – as is the possibility that it is now spreading to people who were never vaccinated. In the laboratory, the virus has been shown to be carcinogenic, and a possible link between the virus and several types of cancer, from brain cancer to lymphoma, has been investigated, but there isn’t yet definitive evidence either way. Nevertheless, it suddenly became necessary to find an alternative supply of cells."