Transfer of vaccinia (smallpox vaccine), a live and highly dermatotrophic virus, from the vaccination site to the skin and mucosa is a common adverse event of smallpox vaccination, due to the tendency to scratch the itching vaccination site that contains the live vaccinia virus (or, less commonly, from handling gauze or other materials that cover or come in contact with the site).

Accidental (inadvertent) implantation can be by autoinoculation or contact with a recent vaccinee and may vary from single lesions to massive involvement of disruptive skin in patients with eczema, acne, skin disorders with open lesions, and inflammatory eye diseases, especially those treated with steroids.

In individuals with cell-mediated immune defects, the implantation can be serious and life-threatening and lead to progressive vaccinia. Although no age group is spared, children are most susceptible to more extensive inoculations because of their propensity to scratch the vaccination site. Adults frequently implant virus on the face as a result of inadvertent contamination of the hands or via fomites.