"In the early days of making the vaccine, researchers infected fetal kidney cells in Petri dishes to produce a large amount of virus that they could then harvest, purify and use to vaccinate people. (The virus evolves to become less deadly when it infects cells out of the body, and thus could safely be given to people to prime their immune system for the real thing.) Today manufacturers of the polio vaccine use other types of human cells, which weren’t available in the mid-1900s. They also use monkey cells, which they originally avoided for fear that making the vaccine in animal cells could put people at risk of diseases from other species."