The following is from the CDC site http://www.bt.cdc.gov/agent/smallpox/overview/faq.asp I just copied and pasted. What is the smallpox vaccine made of? The vaccine is made from a virus called vaccinia, another “pox”-type virus related to smallpox but that does not cause smallpox. The smallpox vaccine helps the body develop immunity to smallpox. It does not contain the smallpox virus and cannot spread smallpox Is it possible for people to get smallpox from the vaccination? No. The smallpox vaccine does not contain smallpox virus and cannot spread or cause smallpox. However the vaccine does contain another virus called vaccinia, which is “live” in the vaccine. Because the virus is live, it can spread to other parts of the body or to other people from the vaccine site. This can be prevented through proper care of the vaccination site (e.g., hand washing and careful disposal of used bandages). Is it possible to get vaccinia, the virus in the vaccine, from someone who has recently been vaccinated? Yes. Vaccinia is spread by touching a vaccination site before it has healed or by touching bandages or clothing that have become contaminated with live virus from the vaccination site. Vaccinia is not spread through airborne contagion. The vaccinia virus may cause rash, fever, and head and body aches. (updated Dec 9, 2002) Live Virus Vaccines and Vaccinia The vaccinia virus is the "live virus" used in the smallpox vaccine. It is a "pox"-type virus related to smallpox. When given to humans as a vaccine, it helps the body to develop immunity to smallpox. The smallpox vaccine does not contain the smallpox virus and it cannot cause smallpox.