What is the process called when a pathogen invades and grows within a host?

The correct answer is "Infection," which refers to the process where a pathogen successfully invades a host and begins to replicate. This process is crucial in understanding how diseases develop. When a pathogen enters the body and overcomes the host's defenses, it can multiply and cause harm, leading to the symptoms associated with infections.

During this process, the immune system responds to the presence of the pathogen, and the outcome can vary from complete recovery to severe illness, depending on various factors such as the pathogen's virulence, the host's immune status, and the presence of other influencing factors.

The other options relate to concepts that are associated but distinct from infection. For example, "Transmission" describes how pathogens are spread from one host to another, while "Immunity" refers to the body's ability to resist or fight off infections. "Colonization" involves the establishment of a pathogen in a host without necessarily causing disease, which is different from an active infection where the pathogen is proliferating and causing symptoms. Understanding these distinctions is important in epidemiology and public health interventions.