cause disease. Inactivated Some vaccines contain inactivated, but previously virulent, micro-organisms that have been destroyed with chemicals, heat, or radiation"ghosts", with intact but empty bacterial cell envelopes. They are considered an intermediate phase between the inactivated and attenuated vaccines. Examples include IPV (polio vaccine), hepatitis A vaccine, rabies vaccine and most influenza vaccines. Toxoid Toxoid vaccines are made from inactivated toxic compounds that cause illness rather than the micro-organism. Examples of toxoid-based vaccines include tetanus and diphtheria. Not all toxoids are for micro-organisms; for example, Crotalus atrox toxoid is used to vaccinate dogs against rattlesnake bites. Subunit Rather than introducing an inactivated or attenuated micro-organism to an immune system (which would constitute a "whole-agent" vaccine), a subunit vaccine uses a fragment of it to create an immune response. One example is the subunit vaccine against hepatitisB, which is composed of only the surface proteins of the virus (previously extracted from the blood serum of chronically infected patients but now produced by recombination of the viral genes into yeast). Another example is edible algae vaccines, such as the virus-like particle (VLP) vaccine against human papillomavirus (HPV), which is composed of the viral major capsid protein. Another example is the hemagglutinin and neuraminidase subunits of the influenza virus. A subunit vaccine is being used for plague immunization. Conjugate Certain bacteria have a polysaccharide outer coat that is poorly immunogenic. By linking these outer coats to proteins (e.g., toxins), the immune system can be led to recognize the polysaccharide as if it were a protein antigen. This approach is used in the Haemophilus influenzae type B vaccine. Outer membrane vesicle Outer membrane vesicles (OMVs) are naturally immunogenic and can be manipulated to produce potent vaccines. The best known OMV vaccines are those developed for serotype B meningococcal disease. Heterotypic Heterologous vaccines also known as "Jennerian vaccines", are vaccines that are pathogens of other animals that either do not cause disease or cause mild disease in the organism being treated. The classic example is Jenner's use of cowpox to protect against smallpox. A current example is the use of BCG vaccine made from Mycobacterium bovis to protect against tuberculosis. Genetic vaccine The subgroup of genetic vaccines encompass viral vector vaccines, RNA vaccines and DNA vaccines. Viral vector Viral vector vaccines use a safe virus to insert pathogen genes in the body to produce specific antigens, such as surface proteins, to stimulate an immune response.