(CNN)Here's some background information about smallpox, a contagious and sometimes fatal infectious disease. The most severe and most common form of smallpox is known as variola major, which has four variants.
Ordinary - most common, 90% of cases.
Modified - mild and occurs in those having been vaccinated.
Flat Hemorrhagic - the most rare, severe and usually fatal.
Smallpox is transmitted through extended face-to-face contact, or direct contact with infected body fluids or contaminated objects.
In people exposed to smallpox, the vaccine can lessen the severity of or even prevent illness if given within three to four days after exposure.