Dose Study of ACAM2000 Smallpox Vaccine in Previously Vaccinated Adults

Inclusion criteria: * have a history of previous smallpox vaccination greater than 10 years previous to the date of screening and presence of a vaccination scar. * females must not be pregnant, lactating and must agree to use an effective form of birth control for the 30 days following vaccination or unable to bear children. * agree to be available for the entire study and agree to comply with all requirements. exclusion criteria: * children 1 year of age or younger in the household or be in close contact * smallpox vaccination within ten years * known or suspected Human immunodeficient virus (HIV) infection, primary immunodeficiency disorder, leukemia, lymphoma, or current radiation treatment or use of immunosuppressive or anti-neoplastic drugs or have a household member or intimate contact with the conditions listed above. * renal disease * current or past history of eczema or a household member or direct contact who has eczema. * known allergy or past allergic reactions to latex gloves or to antibiotics which include neomycin, streptomycin, chlortetracycline, and polymyxin B * known allergy or past allergic reaction to blood products. * known allergy to cidofovir or sulfa-containing drugs. * history of allergic phenomena following smallpox vaccination in the past, including urticaria, erythema multiforme, or Stevens-Johnson syndrome. * transfusion of blood or treatment with any blood product. * current or history of drug or alcohol abuse * innoculation with any other live vaccine or participating in another drug or vaccine trial within 30 days of enrollment.