Pathogen
A pathogen is an agent which causes disease [1], such as microorganisms like bacteria (for example, Salmonella) or fungi (such as the plant pathogen Aspergillis niger).
An infectious agent that causes disease. It could be a bacterium or a virus.
A pathogen causes disease by gaining entry to the host (such as through the epithelial layer of the small intestines when eating certain food) and damaging the tissues of the host.