There have been a number of well-documented instances of contaminated vaccines transmitting viruses to humans, such as batches of poliovirus and adenovirus vaccines that were found to be contaminated with Simian vacuolating virus 40 (SV40) from rhesus monkey kidney cells in the late 1950s and early 1960s; and some early batches of yellow fever vaccine produced in hens' eggs were found to be contaminated with avian leucosis virus.