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.
Examples of Viruses are hepatitis A, B and C [2]