A pathogen is any organism that causes disease. Pathogens include microorganisms and some larger organisms such as tapeworms. Pathogenic microorganisms include some bacteria, some fungi and all viruses.
Pathogens can penetrate an organisms interface with the environment. Pathogens need to enter the body in order to cause disease, they get it through an organism surface of contact (interface) with the environment, eg-nose, eyes or a cut.
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