When a virus infects a cell, it must hijack that host cell’s inner machinery, normally used to manufacture necessary molecules for the host cell, and divert that machinery to producing new viruses. Previous research has indicated that arginine, an amino acid, plays an important role in viral infection. We investigated the role arginine plays in infection in two ways. First, we compared how well bacteriophage, a type of bacteria-infecting virus, replicated in normal (parent) E. coli and genetically modified E. coli that could not produce their own arginine. These genetically modified E. coli are called a knock-out strain because the gene for a particular protein, in this case an enzyme involved in producing arginine, is removed. The gene in question is called argH and thus the knock-out strain is named ΔargH. Here we found that when arginine was available from outside the cell, there was no significant difference between bacteriophage replication in the two E. coli strains. Second, we observed how the levels of certain small molecules (metabolites), including arginine, inside a human cell changed after it was infected with the Human Cytomegalovirus (HCMV). We found that HCMV infected cells had altered levels of metabolites from throughout the arginine biosynthesis pathway, including increased levels of arginine.