 Participants are randomly assigned to receive injections of one of the following: 1 study vaccine at 1 mg dose, 2 study vaccine at 4 mg dose, or 3 placebo salt-water solution. They receive three injections about 4 weeks apart in the upper arm muscle. Participants record their temperature and symptoms at home for 5 days after each injection, either on a diary card or electronically using the Internet, and report any side effects to a study physician or nurse as soon as possible. They return to NIH for clinic visits every 2 weeks for the first 12 weeks, then at week 26 and at week 42 to check for health changes or problems. Blood is drawn at all visits and urine samples are collected through week 10. 