AMSA Basic Nursing 103 Practice Test

The chain of infection begins with the infectious agent—the microorganism (bacteria, virus, fungus, or parasite) capable of causing disease. If this pathogen isn’t present, infection cannot start, so it is the first link in the chain. After the infectious agent come the reservoir where it can grow, the portal of exit how it leaves that reservoir or host, the mode of transmission, the portal of entry into a new host, and finally a susceptible host. The other options describe later steps in the process, not the initial starting point. This is why the infectious agent is the correct choice.