Sub-unit vaccines are made from parts of infectious agents, or certain chemicals produced by bacteria. Because the vaccine does not contain whole organisms, they cannot cause disease in immunized people. The diphtheria and tetanus components of the pentavalent vaccine are of the sub-unit type. Diphtheria and tetanus bacteria each produce special toxins — harmful chemicals that cause the symptoms of these diseases. The pentavalent vaccine contains diphtheria and tetanus toxoids — modified versions of the bacterial toxins, which have been developed in a laboratory. The toxoids don’t cause disease symptoms, but they do stimulate a protective immune response in vaccinated people. A sub-unit version of the pertussis vaccine now exists and is increasingly being used instead of the older whole-cell inactivated version.