Antiserum (plural: antisera) is blood serum containing polyclonal antibodies. Antiserum is used to pass on passive immunity to many diseases. Passive antibody transfusion from a previous human survivor is the only effective treatment for Ebola infection.
Antibodies in the antiserum bind the infectious agent. The immune system then recognizes foreign agents bound to antibodies and triggers a more robust immune response. The use of antiserum is particularly effective against pathogens which are capable of evading the immune system in the unstimulated state but which are not robust enough to evade the stimulated immune system. The existence of antibodies to the agent therefore depends on an initial "lucky survivor" whose immune system by chance discovered a counteragent to the pathogen, or a "host species" which carries the virus but does not suffer from its effects. Further stocks of antiserum can then be produced from the initial donor or from a donor organism that is inoculated with the pathogen and cured by some stock of preexisting antiserum.
Mutations in the pathogen can decrease or eliminate the effectiveness of antiserum.