Immunology is the science of the body's defence system: how we notice germs, fight them, remember them, and sometimes stop a disease before it starts. Vaccines, allergies, and illnesses where the body attacks itself all sit in this field. The learner-dictionary line says it is the study of preventing disease and of how the body reacts to disease, and that is the door in — though actually it is narrower than all of 'preventing disease'. Washing your hands is health advice, not immunology; a vaccine is immunology, because it teaches the defence system. People who work in it are immunologists. It is a formal science word, for textbooks, labs, and hospital reports, not for chatting about a cold at dinner — even though a cold is exactly the kind of fight this science studies.