Variola major is the most severe form of smallpox and can be further classified into five clinical types on the basis of differences in rash characteristics and density. The prognosis differs among the types (Fenner 1988). The clinical types are:
Ordinary (or classic) smallpox
Flat-type (or malignant) smallpox
Hemorrhagic smallpox
Modified smallpox
Variola sine eruptione
In the pre-eradication era, diagnosing smallpox and distinguishing its type took into account clinical illness pattern, epidemiologic considerations, and laboratory findings. Although there is some overlap between ordinary, flat-type, and hemorrhagic smallpox, their clinical and epidemiologic features are sufficiently distinct to warrant separate consideration (see below), particularly to enhance clinicians’ awareness of the various clinical manifestations of what should be an extinct disease.
Modified smallpox was like ordinary smallpox but had an accelerated course and was a milder illness with fewer skin lesions and a low case-fatality rate; it was more likely to occur in persons with some immunity from past vaccination. Variola sine eruptione occurred in vaccinated contacts of cases and was characterized by sudden onset of fever, headache, and backache. Illness resolved in 1 to 2 days without development of a rash.