Describe the 3 types of smallpox

This is what I can gather from the text of the book:

It comes in two natural subspecies, variola minor and variola major. Minor is a weak strain that was first identified by doctors in Jamaica in 1863, and is also called alastrim. While it causes people to pustulate, for some reason it rarely kills. Variola major kills around twenty to forty percent of infected umans who are not immune to it, depending on the circumstances of the outbreak and how virulent, or hot, the strain is. As a generality, doctors say that smallpox kills one out of three people. Virus particles are also known as virions. Smallpox virions are very small. About one thousand of them would span the thickness of a human hair. It may be that you can catch smallpox if you inhale three to five infectious virions, or particles. No one knows the infectious dose of smallpox, but experts believe it is quite small.