The Infectious Agent is the micro-organism (such as bacteria, fungi and parasites) which cause diseases.
The Reservoir is the place where the infectious agent (micro-organism) can live, grow and re-produce. Examples of a reservoir are people, animals, water and food.
The Means of exit is the way the infectious agent leaves the reservoir. Examples of this include blood, excretions (faeces and vomit) and secretions (mucus, saliva, tears and bile).
The Mode of transmission is how the infectious agents are transmitted from one person to another. This happens through physical contact with another person or object, through airborne droplets (e.g. coughs, sneezes) or contact with bodily fluids.
The Portal of entry is how the infectious agents enters the body of a person. Pathogens can enter the body by being breathed in or eaten, coming into contact with broken skin, eyes, nose or mouth.
The Person at risk is someone who has increased vulnerability to infection. For example, elderly people, infants and those who are immunocompromised.