This study is a single (Part A) and multiple (Part B) dose study. Following informed consent and screening, 10 eligible subjects will receive a single dose of Thetanix or placebo in the clinic. A Safety Review Committee will review the safety data up to Day 7 from these first ten subjects and determine if it is appropriate to continue into Part B in which 10 subjects will receive 15 doses of Thetanix or placebo in a twice daily dosing regimen over 7.5 days. In both parts of the study, 8 of the 10 subjects will randomly receive Thetanix and 2 subjects will randomly receive placebo. Each dose consists of three capsules. Subjects in Part B will receive the first and last dose in the clinic and will take 13 doses at home. While at home, subjects will be required to answer questions about their health, record their body temperature and when they take the capsules in an electronic diary. Subjects developing a fever will undergo further assessments, including blood cultures. Subjects will provide stool samples for analysis of microbiota and faecal calprotectin.