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Background: - Malaria parasites are carried by mosquitoes, which spread the infection by biting people. Currently, there is no effective malaria vaccine. However, studies show that volunteers bitten many times by mosquitoes that carry weakened malaria parasites could fight off getting sick with malaria when later exposed to normal malaria parasites. Malaria parasites are weakened by exposing them to radiation when they are in the stage of development called sporozoites . Only the mosquitoes are irradiated and study volunteers are not exposed to radiation. The radiation stops the parasites from being able to cause disease but still promote protection. For many years, it was not possible to give these sporozoites to people as a vaccine since they could not be adequately purified from the mosquito. Scientists have recently figured out how to produce and isolate the weakened sporozoites so that they can be given in an injected vaccine. This vaccine is known as the "PfSPZ vaccine". - A malaria challenge will be used to test whether the vaccine will prevent infection. In a malaria challenge, mosquitoes that have the malaria parasite will be allowed to bite a participant's arm. In the event that the vaccine does not work, the malaria parasite used for the challenge can be treated completely with common anti-malaria medications. Participants will be treated immediately if they develop malaria symptoms. Objectives: - To test the safety and effectiveness of the PfSPZ vaccine. Eligibility: - Healthy volunteers between 18 to 45 years of age. Design: - Participants will be screened with a physical exam, medical history, and blood tests. There will be five different groups of study participants, all of whom will be monitored with frequent blood tests. - Group 1 will have two vaccines with the lowest amount of the vaccine given 4 weeks apart, with regular clinic visits up to 24 weeks after the second vaccine. This group will not have a malaria challenge. - Group 2 will have four or six vaccines given 4 weeks apart at a higher dose than group 1. A malaria challenge will be given about 3 weeks after the last vaccine. Follow-up visits will continue through 24 weeks after the last vaccine. - Group 3 will have four or six vaccines given 4 weeks apart at a higher dose than group 2. A malaria challenge will be given about 3 weeks after the last vaccination, as for Group 2. Follow-up visits will continue through 24 weeks after last vaccine. - Group 4 will have four or six vaccines given 4 weeks apart at a higher dose than group 3. A malaria challenge will be given about 3 weeks after the last vaccination. Follow up visits will continue through 24 weeks after last vaccine. - Group 5 will serve as a control group and will not receive the vaccine, but will have the malaria challenge. Follow-up visits will continue through 8 weeks after the challenge. All participants from any group who receive a malaria challenge will be treated promptly for malaria when it develops.