This is a first-in-human, Phase 1/1b, 4-part study that includes the evaluation of safety, tolerability, pharmacokinetics (PK), and immunogenicity of BCX17725 when administered via single and multiple doses in healthy adult participants (Parts 1 and 2), and multiple doses in adult participants with Netherton syndrome (Part 3). In Part 4, the effectiveness, safety, and tolerability of BCX17725 when administered via multiple IV and/or SC doses through 12 weeks will be evaluated in adult and adolescent participants with Netherton syndrome.