Final answer:
Dmitri Ivanovsky is credited with advancing the field of virology by discovering the existence of viruses, which are smaller than bacteria and could pass through bacteria-trapping filters.
Step-by-step explanation:
The microbiologist credited for advancing the field of virology is Dmitri Ivanovsky. While Louis Pasteur and Robert Koch made significant contributions to the field of microbiology and the germ theory of disease, it was Ivanovsky who, in 1892, first discovered viruses as infectious agents which were smaller than bacteria and passed through filters that were known to trap bacteria. Pasteur and Koch, on the other hand, were groundbreaking in developing vaccines, identifying specific bacteria causing diseases, and disproving the spontaneous generation hypothesis. Hence, for the field of virology specifically, Dmitri Ivanovsky's work laid the foundation.