Infectious diseases occur when pathogenic microorganisms, such as bacteria, viruses, fungi, or parasites, invade the body, multiply, and disrupt normal cellular functions. At the cellular level, these pathogens can damage host cells through direct invasion, toxin production, or triggering inflammatory responses. For instance, bacteria may release exotoxins that interfere with cellular signaling or induce apoptosis. Viruses invade host cells and hijack their machinery for replication, often leading to cell lysis. The immune response, which includes the activation of T cells, B cells, and the secretion of cytokines, aims to eliminate the pathogens but can also cause collateral damage to healthy tissues. Chronic infections can lead to persistent inflammation and tissue remodeling, contributing to long-term complications. Understanding these mechanisms is crucial for effective prescribing and management of infectious diseases.