Health officers have recognized this in continuing the quarantine against the disease until the organism disappears.

In typhoid fever bacilli may remain in the body for a long time and be continually discharged, as in the well-known case of "typhoid Mary."[1] Single cases of certain infectious diseases may appear in a community year after year, and at intervals the cases become so numerous that the disease is said to be epidemic.

Such a disease is smallpox.

This is a highly infectious disease, towards which all mankind is susceptible.

Complete protection against the disease can be conferred by Jenner's discovery of vaccination.