In children who have extremely severe and recurring infections with common pathogens or infections with rare pathogens, physicians should investigate a primary immunodeficiency condition. Physicians should be aware that some conditions can present with similar symptoms to those of PID. These include cancer, chronic liver disease, and autoimmune disorders such as lupus erythematosus and inflammatory bowel disease. Children with primary immunodeficiencies are at risk for serious infection due to exposure to pathogens not encountered in other people. The severity of these infections can vary greatly, from mild fever to death. Some patients may experience multiple episodes of infection per year; others may remain free of illness for many years before developing an immune deficiency. Immunodeficient individuals can be infected with organisms not normally associated with illness. For example, HIV-infected individuals may develop infection with bacteria like Mycobacterium tuberculosis or C. difficile. Patients with primary immunodeficiencies are also at risk for infection with viruses like the human herpesvirus 8 (HHV-8) that can cause lymphoma and other cancers. Primary immunodeficiencies can be classified into three main categories: cellular deficiencies, genetic mutations, and dysregulated signaling pathways.