Vaccines in which purified immunogens capable of eliciting protective response are identified and selectively used in isolation are referred to as subunit vaccines. Biotechnology has provided this new class of vaccines called subunit vaccines. The current tendency is to use genetic engineering techniques for this purpose. This type of vaccine is created by using purified antigen subunits, which are produced through r-DNA technology. Genes that code for the desired antigen in the immunization process are introduced into the non-pathogenic organism. A subunit vaccine has no potential for reverting to a pathogenic organism because only limited numbers of components from original pathogen are present in the vaccine. A subunit vaccine should also have fewer side effects as it does not have any infectious agent. Live attenuated vaccines have been derived empherically, usually after extended passage on artificial medium or on tissue culture. Biotechnology now starts with the analysis of a pathogen’s virulence and antigenicity at the molecular level. This enables a more rational; approach to the generation of attenuated organisms. Improving attenuation improving stability and improving immunogenicity of vaccines has been possible with new techniques. Currently available live attenuated vaccines may be used as hosts for foreign genes. A gene choice coding for a protective antigen is expressed by the live vaccine. Upon vaccination with the recombinant, immune response to the carrier vaccine and to the foreign gene is generated. It is hoped that they are sufficient to elicit immune response against both the pathogen from which the foreign gene has been derived and against the parent vaccine. The 1995 Biotechnology Medicines in Development, published by Pharmaceutical Research and Manufacturers of America, listed 40 different vaccines at various stages in clinical trials. Vaccines in phase III clinical trial are for diphtheria , pertusis, tetanus, herpes, HIV , Lyme disease and melanoma.  However such purification of antigens is potentially dangerous and inefficient. Therefore there is considerable impetus to isolate the microbial host like E.coli, B.subtilis or yeast.