(CNN) -- Here's a look at what you need to know 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.
Types of variola smallpox: Ordinary - most common, 90% of cases. Modified - mild and occurs in those having been vaccinated. Flat Hemorrhagic - the most rare, severe and usually fatal.
Facts: Smallpox is transmitted through extended face-to-face contact, or direct contact with infected body fluids or contaminated objects.
Insects and animals do not transmit smallpox. If aerosolized smallpox were to be released, 90% of the virus would die within 24 hours.
The incubation period is about 12 days (ranges from 7 to 17 days) following exposure. Subjects are not contagious at this time.
Initial symptoms include high fever, fatigue, and head and back aches. A characteristic rash, most prominent on the face, arms, and legs, follows in 2-3 days. The rash starts with flat red lesions that evolve at the same rate. Lesions become pus-filled and begin to crust early in the second week. Scabs develop and then separate and fall off after about 3-4 weeks.
The majority of patients with smallpox recover, but death occurs in up to 30% of cases.
1950s - Worldwide, 15 million cases of smallpox are reported each year in the decade; the highly contagious disease kills more than 500 million people worldwide during the last century.
1977 - The last naturally occurring case of smallpox in the world occurs in Somalia.
1979 - Smallpox meets the criteria for eradication by having no natural cases for two years, and is officially eradicated in May 1980. There are two sanctioned repositories for stocks of variola, the virus that causes smallpox. They are: the Centers for Disease Control and Prevention in Atlanta, Georgia, and the Russian State Research Center of Virology and Biotechnology in Koltsovo.
2014 - Six vials containing the smallpox virus are found in an unused storage room at the Food and Drug Administration's Bethesda, Maryland, campus. Later testing shows that at least two of the vials, dating from 1954, contain the live virus.
Smallpox Vaccine: In people exposed to smallpox, the vaccine can lessen the severity of or even prevent illness if given within 3-4 days after exposure.
Vaccine against smallpox contains a live virus called vaccinia, which is related to the variola virus that causes smallpox.
Exposure to the vaccinia pathogen in the vaccine can cause severe complications in rare cases. The people most at risk are those with compromised immune systems, like those with HIV or people undergoing cancer treatment. Also at risk are people with certain skin conditions who may be more sensitive to the virus in the vaccine.
Most Americans under 40 have not been vaccinated. The last smallpox case in the United States was in 1949, and routine vaccination stopped in 1972. Some medical and military personal are still vaccinated.