In case where the host displays a protective immunological response such that resistance to new infection will be enhanced and/or the clinical severity of the disease reduced, the immunogenic composition is described as a "vaccine".
An "antigen" as used herein refers to, but is not limited to, components which elicit an immunological response in a host to an immunogenic composition or vaccine of interest comprising such antigen or an immunologically active component thereof. The antigen or immunologically active component may be a microorganism that is whole (in inactivated or modified live form), or any fragment or fraction thereof, which, if administered to a host, can elicit an immunological response in the host. The antigen may be or may comprise complete live organisms in either its original form or as attenuated organisms in a so called modified live vaccine (MLV). The antigen may further comprise appropriate elements of said organisms (subunit vaccines) whereby these elements are generated either by destroying the whole organism or the growth cultures of such organisms and subsequent purification steps yielding in the desired structure(s), or by synthetic processes induced by an appropriate manipulation of a suitable system like, but not restricted to bacteria, insects, mammalian or other species, and optionally by subsequent isolation and purification procedures, or by induction of said synthetic processes in the animal needing a vaccine by direct incorporation of genetic material using suitable pharmaceutical compositions (polynucleotide vaccination). The antigen may comprise whole organisms inactivated by appropriate methods in a so called killed vaccine (KV). If the organism is a bacterium, the killed vaccine is called a bacterin.