This study is directed to patients with known or suspected Strongyloides stercoralis infection because it is a relatively common parasitic infection, even in the United States. It is difficult to diagnose, and efficacy of treatment is difficult to evaluate. Some infected individuals can develop serious even fatal, disease under certain conditions of immunosuppression. Because newer diagnostic methods are needed to diagnose this infection, we have developed new diagnostics that will be evaluated in comparison to more standard diagnostic tests. Serum and cells will also be collected from patients on this protocol to understand the cellular and humoral response to the parasite and its antigens. All subjects proven to have Strongyloides stercoralis infection will be treated with standard therapy and followed to assess both the efficacy of treatment and the changes in humoral and cellular immune responses induced by treatment.