Final answer:
A live virulent vaccine contains a fully virulent, disease-causing pathogen, which is very risky to use and not commonly practiced due to the high chance of causing disease. Live attenuated vaccines, which contain weakened pathogens, are more commonly used.
Step-by-step explanation:
The term that best defines a vaccine containing the fully virulent disease-causing agent is a live virulent vaccine. Unlike subunit, recombinant, or inactivated vaccines, a live virulent vaccine contains a disease-causing pathogen that has not been weakened or killed. However, using a live virulent vaccine is extremely risky and not a common practice, as it poses a high risk of causing the disease it is meant to prevent. Generally, live attenuated vaccines are used instead, where the pathogen is weakened and unable to cause disease under normal circumstances. The danger with live attenuated vaccines is the potential for the pathogen to revert to a virulent form through back mutations. Subunit vaccines and recombinant vaccines are safer alternatives since they contain only parts of the pathogen or are produced using genetic engineering to create specific antigenic components.