Co-stimulatory T-cell inhibitors are used in the treatment of rheumatoid arthritis and to prevent rejection of renal transplants. Inhibitors of the interleukin (IL)-17 cytokine are indicated for psoriasis, psoriatic arthritis and ankylosing spondylitis and anti- IL-23 drugs for psoriasis. Serious infections occur in 4.2% to 25.0% of co-stimulatory inhibitors and 1.0% to 2.0% with IL-17 or IL-23 inhibitors. Underlying disease, steroid dose greater than 7.5 to 10.0 mg, and comorbidities influence risk in individual patients. Opportunistic infections or reactivation of tuberculosis are rare.