Induced Blood-Stage Malaria in Healthy Malaria-Naive Adults to Assess the Safety and Infectivity of Plasmodium Vivax Challenge Agent and Evaluate Transmission in Mosquito Feeding Assays

Background: Malaria is a disease caused by parasites transmitted to people by mosquitoes. Around the world, there were 241 million cases and 627,000 deaths from malaria in 2020. Researchers are working to develop vaccines and treatments for this disease. Objective: To learn how malaria develops in people; how the body's immune system reacts to malaria; and how malaria spreads from people to mosquitoes. Eligibility: Healthy people in the Washington DC area, aged 18 to 54 years. They cannot live alone during parts of the study. Design: Participants will be infected with a parasite that causes malaria. The parasite will be in donated blood; it will be given through an IV. Participants will likely develop symptoms within a week after the injection. Researchers will call daily to check on their health. After about 6 days, participants will come to the NIH clinic each day for blood tests. Participants will check in to the NIH clinic around 10 days after the injection. They will stay in the clinic 3 to 6 days. They will have multiple blood tests every day. Participants will be bitten by mosquitoes up to 4 times. Cups containing mosquitoes will be held against their skin for 15 minutes. Participants will begin taking chloroquine close to the end of their clinic stay. Chloroquine is a pill taken by mouth once or twice a day for 3 days. It is FDA-approved to treat malaria. Participants will have follow-up visits 1 and 3 weeks after discharge.

* INCLUSION CRITERIA: All of the following criteria must be fulfilled for a participant to undergo IBSM: 1. Age \>=18 and \10% cardiovascular risk as determined by the non-laboratory method or an abnormal EKG demonstrating a corrected QT interval by Fridericia's formula of \>450 msec or other concerning arrhythmia. 4. Any other medical condition that may have significant implications for current health status and participation in the study, in the opinion of the investigator. 10. History of a severe reaction to arthropod bites, or history of anaphylaxis or severe unexpected allergy to any substance. 11. Screening blood test or urinalysis laboratory parameters outside of local lab normal range (including infectious serologies). Participants may be included at the investigator's discretion for "not clinically significant" values outside of normal range. 12. Any other finding that, in the judgment of the investigator, would interfere with, or serve as a contraindication to, protocol adherence, assessment of safety or reactogenicity, or a participant's ability to give informed consent, or increase the risk of having an adverse outcome from participating in the study. Participants who are determined ineligible to participate for any of the reasons above may be rescreened for eligibility at a later time when the disqualifying condition may be resolved.