Ustekinumab is a human monoclonal antibody typically used to treat moderate to severe plaque psoriasis, psoriatic arthritis, moderate to severe Crohn's disease, or moderate to severe ulcerative colitis (inflammatory bowel disease). Ustekinumab mediates the body's T-cell response by acting as an antagonist against interleukin-12 (IL12) and interleukin-23 (IL23). Although rare, the emergence of severe infections or exacerbation/reactivation of existing infections (bacterial, mycobacterial, fungal, viral) is possible for Ustekinumab. We report a case of a 29-year-old female patient who was prescribed Ustekinumab for Crohn’s disease management. After the commencement of the drug for two doses she developed Varicella pneumonia with ARDS which was subsequently managed.