A person may be able to join this study if they are between 15 and 40 years old and have sickle cell disease type SS or beta zero thalassemia. They must have been stable for at least two weeks, with no recent sickle cell pain crisis, hospital stay, or new health problem needing medical care. They cannot be taking zinc supplements, or they must be able to stop them for three months before the study begins. They must be able to swallow pills, take a zinc pill every day, and follow the study visits and surveys. Some people cannot join the study. These include people who have very low vitamin D, severe kidney disease, are in another research drug study, have certain transfusion or iron overload problems, or have just started or changed some sickle cell medicines like hydroxyurea, L glutamine, or crizanlizumab.