Mutation: In biology, mutation refers to any change in DNA sequence away from normal. Mutations may be caused by mistakes during cell division or exposure to DNA-damaging agents in the environment.
Pathogen: A pathogen is anything that can produce disease. Typically the term is used to mean an infectious agent - a microbe or microorganism such as a bacterium, virus, fungal or protozoan that causes disease in its host.
Viral Load: This refers to the amount of virus in an infected person's blood. It is often used as a marker of how severe a viral infection may be or how quickly the infection could spread.