All ages are equally susceptible to smallpox, and yet in the general prevalence of the disease in the prevaccination period it was almost confined to children, the adults being protected by a previous attack.

The habits and environment at different ages have an influence on the opportunities for infection.

There is comparatively little opportunity for infection during the first year, in which period the infant is nursed and has a narrow environment within which infection is easily controlled.

With increasing years the opportunities for infection increase.

When the child begins to move and crawl on hands and knees the hands become contaminated, and the habit of putting objects handled into the mouth makes infection by this route possible.