(CNN) Here's a look at smallpox, a contagious and sometimes fatal infectious disease. The most severe and most common form of the smallpox virus is known as variola major.

Types of variola major smallpox

Ordinary - most common, 85% of cases

Modified - mild and occurs in those that have been vaccinated

Flat - occurs more frequently in children and is usually fatal

Hemorrhagic - occurs more frequently in adults; is severe and usually fatal

Facts

Insects and animals do not transmit smallpox.

Initial symptoms include high fever, fatigue and head and back aches. A characteristic rash that is prominent on the face, arms and legs follows in two to three 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 three to four 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.

1977 - The last naturally occurring case of smallpox in the world occurs in 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.

1980 - The World Health Organization announces the official eradication of smallpox.

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. There are only two sanctioned repositories for stocks of variola, the virus that causes smallpox, the Centers for Disease Control and Prevention in Atlanta and the Russian State Research Center of Virology and Biotechnology in Koltsovo.

2016 - The oldest known sample of the smallpox-causing The oldest known sample of the smallpox-causing variola virus is found within the DNA of a 17th century child mummy , in a crypt beneath a Lithuanian church, according to a study in the journal Current Biology. The finding shortens the timeline for how long smallpox may have afflicted humans.

Smallpox Vaccine

In people exposed to smallpox, the vaccine can lessen the severity of or prevent illness when given within three to four days after exposure.

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 and 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 personnel are still vaccinated.