CNTO 1959 is an experimental drug. In this study, CNTO 1959 will be tested to see if it may be useful in treating psoriasis. This disease causes inflammation of skin and nails. This is a Randomized (study drug assigned by chance), Double-blind (neither physician nor patient knows the name of the assigned drug), Placebo-controlled study of the experimental drug CNTO 1959 in both healthy patients as well as patients with psoriasis. This study will compare the effects (both good and bad) of CNTO 1959 to those of placebo. The purpose of this study is to evaluate the safety and potential side effects of CNTO 1959 given to healthy volunteers and patients with moderate to severe psoriasis. In addition, the investigators will measure how much study drug is in the blood following a dose of CNTO 1959, and determine if the body makes antibodies to it. (An antibody is a protein made by the body in response to a foreign substance). Safety assessments will be performed throughout the study and include obtaining and evaluating laboratory tests, vital signs (e.g., blood pressure), and the occurrence and severity of adverse events. This is the first time that CNTO 1959 will be studied in humans. There will be 2 parts to this study. The first part of this study will involve healthy volunteers, and the second part of the study will involve patients with psoriasis. About 47 healthy volunteers and 24 patients with moderate to severe psoriasis will take part in the study. Enrollment for the healthy volunteer part is closed as of 23SEP09. CNTO 1959 will be administered as either an intravenous (IV) infusion or subcutaneous (SC) injection. Duration of participation may be up to 28 weeks.