Final answer:
A virus carrying a human transgene to a yeast cell acts as a viral vector, which is a tool for transferring genetic material into a cell without causing disease.
Step-by-step explanation:
When a human transgene is carried into a yeast host cell via a virus, the virus is serving as a viral vector. Viral vectors are modified versions of viruses that have been engineered to carry genetic material without causing disease. They are commonly used in genetic engineering and gene therapy to transfect eukaryotic cells, by facilitating the transfer of DNA into the host cell's genome, thereby potentially altering the host cell's properties or functions. This technology has applications in various areas, including medicine, where it can be used to introduce healthy genes into patients suffering from genetic disorders.