This document discusses infectious arthritis and osteomyelitis, including causative pathogens and laboratory diagnostics. It describes how acute bacterial arthritis is usually caused by Staphylococcus aureus or Streptococcus and can result from hematogenous spread or direct inoculation. Acute viral arthritis causes polyarticular symptoms along with systemic viral infection, often due to parvovirus B19, rubella virus, or hepatitis. Synovial fluid analysis examines cell count and presence of crystals or microbes. Osteomyelitis definitions, classifications, and typical pathogens are provided, noting Staphylococcus aureus as a common cause. Diagnostic tests and treatment approaches are outlined for different clinical scenarios of osteomyelitis.