Comorbidity refers to the co-occurrence of multiple disorders in an individual. For example, schizophrenia often co-occurs with substance use disorders. Systems theory views problems as arising from interactions within a system, such as a family, rather than focusing solely on individuals. The document discusses a case study of a mother, Leah, and her two children who were referred for case management services and housing assistance due to issues with her boyfriend. It provides an overview of systems theory and its key principles for taking a holistic, descriptive approach to understanding problems in relation to wider social systems.