Inoculate

To protect a person or animal from disease by putting a small, safe amount of a virus or bacteria into their body so it can learn to fight it.

From Latin 'inoculare' meaning 'to graft a bud into a plant', from 'in-' (in) + 'oculus' (eye, bud). The medical sense grew from the idea of 'grafting' a tiny bit of disease into the body to grow protection.