Study of Tecovirimat for Human Mpox Virus

Inclusion Criteria (All participants; Arms A, B, and C): 1. Laboratory-confirmed or presumptive human mpox virus (HMPXV) infection. 2. HMPXV illness of <14 days duration immediately prior to study entry. 3. At least one active (not yet scabbed) skin lesion, mouth lesion, or proctitis with or without visible ulcers. 4. Non-pregnant people of reproductive potential must agree to use at least one effective means of contraception when engaging in sexual activities that can result in pregnancy, from the time of enrollment through the end of study participation. 5. Ability to provide informed consent (for those above the legal age of consent and those providing consent for minors) and assent (for those who have reached the age of assent, but not the legal age of consent), as allowed by local ethics committees. 6. For participants to be enrolled/followed remotely, ability and willingness to participate in remote telehealth assessments (i.e., video visits). Additional Inclusion Criteria for Arms A and B: 1. Age ≥18 years at the time of study entry. Additional Inclusion Criteria for Arm C: Participants who meet the above entry criteria who also meet any of the following criteria will be registered to Arm C. 1. Age <18 years at the time of study entry. 2. Those with severe HMPXV disease defined as having one or more of the following conditions: * Suspected or confirmed ocular involvement * Facial lesions on the malar, nose, or eyelid region * Confluent facial lesions * Hospitalization due to HMPXV infection or its complications * Lesions that require surgical intervention including debridement, urinary catheterization or sigmoidoscopy, or lesions extending below the dermis. Those with or without severe disease and with one or more of the following: * Severe immunosuppression * Active skin conditions placing the person at higher risk for disseminated infection * Breastfeeding * Pregnancy * Receipt of potent inducers * Current or planned use of another investigational drug at any point during tecovirimat/placebo dosing that would be predicted to have a significant drug-drug interaction with tecovirimat therapeutics. Exclusion Criteria (All participants; Arms A, B, and C): 1. Prior or concomitant receipt of tecovirimat (e.g., under an alternative access mechanism. 2. Planned initiation of intramuscular cabotegravir/rilpivirine during study drug administration or for two weeks following completion of study drug administration. Participants who were stable on long-acting intramuscular cabotegravir/rilpivirine were allowed to enroll. 3. Participants who, in the judgement of the investigator, will be at significantly increased risk as a result of participation in the study. 4. Participants who require intravenous dosing of tecovirimat.