Pox diseases seem to kill more or less alike, and here is what's known about Smallpox: In fatal cases of ordinary smallpox, death usually occurs between the te

Pox diseases seem to kill more or less alike, and here is what's known about Smallpox: In fatal cases of ordinary smallpox, death usually occurs between the tenth and sixteenth days of the illness. The cause of death from smallpox is not clear, but the infection is now known to involve multiple organs. Circulating immune complexes, overwhelming viremia, or an uncontrolled immune response may be contributing factors.[20] In early hemorrhagic smallpox, death occurs suddenly about six days after the fever develops. Cause of death in hemorrhagic cases involved heart failure, sometimes accompanied by pulmonary edema. In late hemorrhagic cases, high and sustained viremia, severe platelet loss and poor immune response were often cited as causes of death.[24] In flat smallpox modes of death are similar to those in burns, with loss of fluid, protein and electrolytes beyond the capacity of the body to replace or acquire, and fulminating sepsis.[48] https://en.wikipedia.org/wiki/Smallpox https://en.wikipedia.org/wiki/Smallpox In particular this caught my attention, ...overwhelming viremia, or an uncontrolled immune response may be contributing factors... What that probably means is at some point the virus amplifies freely in infected tissues, and may precipitate an extreme feedback loop such as a cytokine storm. That would certainly explain the rapid fatalities suffered by the hemorrhagic form. That mode of death, extreme viral reproduction and immune cascade leading to death is also seen in Hemorrhagic Fevers, and other classically devastating infections.