A pathogen is anything that causes a disease. It is the first link in a chain that causes an infection. Pathogens include: Bacteria (eg, bacterial meningitis) | virus (eg, hepatitis B) | Fungus (eg, an athlete's foot). Humans are in contact with pathogens daily. Most of the time our body's immune system destroys them before they can cause harm. Humans are considered "exposed" when we have been in contact with a pathogen and "infected" when a pathogen has entered the body and resulted in a disease. Whether an exposure results in an infection depends on three factors: (1) Dose - the amount of organisms that enter our body, (2) virulence - the strength of the entered organism and (3) host resistance - the ability of our immune system to fight the infection.