The purpose of this study is to test the safety and immune response to a live attenuated dengue vaccine that could protect people against all 4 types of dengue virus. Live attenuated means that while this vaccine contains 4 live dengue viruses the viruses have been attenuated (weakened) so as not to cause dengue disease in people. Dengue virus is spread to people by mosquitoes and can cause sickness and even death. Seventy-two subjects between the ages of 18-45 years old will be enrolled in this research study at Saint Louis University Center for Vaccine Development. Participants will be randomly assigned to 1 of 4 groups to receive 2 doses of the study vaccine or placebo (inactive substance). Study procedures include: maintaining a diary to record temperature and side effects, physical exam, electrocardiogram (ECG) (measures the activity of the heart), and blood samples. Participants will be involved in study related procedures for about 10 months.