Smallpox was an acute, highly contagious, and frequently fatal disease (killing one-seventh to one-quarter of its victims) but conferring lifelong immunity on survivors. It appeared initially as an infrequent epidemic disease affecting all ages, but as the frequency of epidemics increased, a growing proportion of the adult population acquired immunity to the disease, and smallpox was clearly a childhood disease in the London-born population of the eighteenth century, with children under five the main victims. Among those who survived it, morbidity from smallpox was severe in many cases; victims could be left blind or disfigured for life. Few native Londoners would have survived to adulthood without encountering smallpox.
[Indeed, are there any depictions of smallpox in Blake’s work?]