Smallpox; Population drop from 12 million to 235,000

Smallpox is a contagious, disfiguring and often deadly disease caused by the variola virus. Smallpox is believed to have been around for thousands of years. Few other illnesses have had such a devastating effect on human health and history.

There's no treatment or cure for smallpox. The only prevention is vaccination.

Naturally occurring smallpox was eradicated worldwide by 1980 — the result of an unprecedented immunization campaign. However, interest in the disease remains high because stocks of smallpox virus, kept for research purposes, are stored in two high-security labs — one in the United States and one in Siberia. In addition, it's suspected that other governments or entities may hold stockpiles. This has lead to concerns that smallpox may be used as a biological warfare agent.

Smallpox is a highly contagious viral disease that has two variants. The V major has a 35% mortality rate while the less severe V minor has a 1% mortality rate.