take blood away from organism with illness
grow pathogen from the infected organism on an agar plate to form colonies
put pathogen into another healthy organism, this organism should get same disease
take blood from this ill organism and find the same pathogen
major weapon in preventing viral disease
provide acquired immune response to pathogen
resemble pathogen but do not cause illness
prevent viral replication/assembly
useful against rapidly mutating viruses (HIV)
pathogen is active but weakened, preventing disease
most require single dose