Additional Study Information: The purpose of this study is to evaluate how well tecovirimat works for the treatment of human monkeypox virus infection and if the study drug is safe in people. Tecovirimat is approved by the Food and Drug Administration (FDA) to treat smallpox in adults and children, but its use in this study is considered investigational. Tecovirimat has been given safely to some people with monkeypox in the recent outbreak and is currently being studied in several ongoing studies outside of the US, but none of the studies have shown whether or not this study drug works to treat monkeypox. 530 people will participate in this study and receive an active study drug or placebo. Children, people who are pregnant, people with severe disease or severe skin conditions, people taking certain medications that may lower tecovirimat levels in the blood, and people whose immune systems are weakened will be in a separate group that receives tecovirimat with no possibility of receiving placebo there is no limit to how many people will be assigned to this group. Participation in this study will last about 2 months. If you participate in this study, you may have study procedures including physical exams, blood, and urine sample collection, swabs of the sores on your skin, mouth, vagina, and rectum, recording of daily symptoms and completion of questionnaires about your symptoms. You will also have the option of completely remote participation. Participants who are enrolled/followed remotely will be asked to participate in a telemedicine visit to assess outcomes and to provide any relevant documentation of medical encounters to assess outcomes. You will be paid a maximum of $615 for completing all required study procedures.