Which of the following best describes the term 'infection'?

The term 'infection' is best described as a pathogen invading and starting to grow in a host. This definition highlights the process by which microorganisms, such as bacteria, viruses, or fungi, enter the body, multiply, and potentially disrupt normal bodily functions, leading to disease. Infection encompasses not only the invasion of pathogens but also their establishment and replication within the host's tissues.

In contrast, complete immunity against a pathogen refers to the host’s ability to effectively fight off the pathogen, preventing infection from occurring in the first place. Developing symptoms of a disease without infection would illustrate a different scenario, such as a syndrome or illness caused by factors other than direct pathogen invasion. Lastly, passive survival of a pathogen in the environment describes a state where the microorganism is present but not actively causing infection or growth within a host. Therefore, option A most accurately captures the essence of what constitutes an infection.