👉 Open infection, also known as primary infection, is the initial phase where a pathogen enters and begins to multiply within a host's body. This occurs when microorganisms such as bacteria, viruses, fungi, or parasites breach the body's natural barriers—like the skin or mucous membranes—and establish themselves in tissues or fluids. During this stage, the host's immune system responds to the invader, initiating defense mechanisms that can either contain or allow the pathogen to proliferate. The outcome of open infection determines whether the infection becomes localized or systemic, potentially leading to more severe disease if the pathogen gains a foothold and spreads throughout the body.