Final answer:
Viruses cannot be cultivated in blood agar because they require living host cells for replication. Blood agar is a non-living medium typically used for bacterial culture, not viral replication.
Step-by-step explanation:
Unlike bacteria, many of which can be grown on an artificial nutrient medium, viruses require a living host cell for replication. They are grown in vivo (inside living organisms), in embryonated chicken eggs, or in vitro (outside a living organism in cells in an artificial environment), such as tissue culture. Blood agar does not provide the living host cells that viruses need to replicate. Viruses are not considered living because they are not made of cellular structures, lack cell nuclei, and require a host cell to reproduce. Viruses must be introduced into an environment where they can take over the host cell's machinery to replicate. This is why viruses cannot grow on non-living media like blood agar, which is primarily used for culturing bacteria.