The human body can be infected by a variety of different foreign substances.   These include infections as deadly as a virus, or as common as common as a bacterially infected wound.  Once a human is infected with anything foreign the immune system immediately begins fighting the foreign substance.  However the immune system cannot usually kill the entire foreign cells, a few often survive.  However this number is so few a person can hardly claim to be sick or infected.  Although that person is now a carrier.  A carrier is some one who is infected, or has some of a particular foreign substance in him, yet shows no symptoms.  That person could pass on the pathogen or virus without even realizing it.

Viral infection occurs when a virus enters a host, in this case a human host, and begins to use the host cells to replicate.  The virus will attach it self to a host cell and inject its viral DNA into it.  The viral DNA quickly takes over the healthy host DNA and virus replication begins.  If the virus is not stopped it will take over all the host cells.  

Bacterial infection occurs as a pathogen enters a human.  The pathogen does not use the human cells as a host as the virus does, but it begins to kill them.  A bacterial infection is easier to treat because the bacteria can be killed.  However for a virus to be killed the host cells must be killed also.