A pathogen is any agent that can cause disease in a host organism. The term used to describe the ability of a pathogen to cause disease is virulence. Virulence is determined by several factors, such as the pathogen's ability to enter the host and evade the immune system, the amount of damage it can cause to the host's cells and tissues, and the host's overall health status. Immunity refers to the ability of the host organism to resist infection by a pathogen, antigenicity refers to the ability of a pathogen or its components to provoke an immune response, and toxicity refers to the ability of a substance to cause harm or damage to a living organism.