A Study to Assess Concentration of TREMFYA in Breast Milk of Lactating Women Who Are Receiving TREMFYA Therapeutically

Inclusion criteria: * Has an active diagnosis of at least one approved indication for guselkumab (psoriasis, psoriatic arthritis \[PsA\], UC and CD) as confirmed by medical records * Be medically stable on the basis of medical history review performed at screening. Any abnormalities must be consistent with the underlying illness in the study population and this determination must be recorded in the participant's source documents and initialed by the investigator * Currently is on established guselkumab maintenance therapy, that is, has received at least 2 guselkumab subcutaneous (SC) maintenance doses before Day 1 * Has made the decision to be treated with guselkumab and to breastfeed independently prior to the participant consenting to participate in this study * Must be at least 5 weeks postpartum on Day 1 * Have well-established lactation; participant must be exclusively breastfeeding their infant(s) (or not providing more than 1 supplemental bottle of formula/day) when enrolled in the study * Must plan to continue breastfeeding throughout the duration of the study Exclusion criteria * Has any current or previous illness that, in the opinion of the investigator, might confound the results of the study or that could prevent, limit, or confound the protocol specified assessments * Has history of drug or alcohol abuse according to the Diagnostic and Statistical Manual of Mental Disorders, 5th Edition (DSM-V) criteria within 1 year before screening * Uses or has used an investigational drug (including investigational vaccines) or used an invasive investigational medical device within 1 month before enrolling in the study * Has received or plans to receive any live, attenuated vaccine within 12 weeks prior to administration of guselkumab. Non-live vaccines approved or authorized for emergency use (for example, Coronavirus disease-19 \[COVID-19\]) by local health authorities are allowed * Has a positive urine pregnancy test on Day 1