An infection is the invasion and multiplication of pathogenic microorganisms—such as bacteria, viruses, fungi, or parasites—in the tissues of a host organism, which can result in disease. The process begins when the invading agents breach the body’s natural barriers and defenses. The host immune system initiates a complex inflammatory and systemic response intended to contain and eliminate the pathogens. Diagnosis relies on identifying the specific causative organism and assessing the extent of tissue damage, while treatment strategies aim to manage symptoms, support the immune system, and utilize specific agents to curb pathogen proliferation.