Final answer:
They are necessary for viral entry and replication and define the host and cell specificity of the virus. This process is critical to understanding viral infections such as HIV and influenza.
Step-by-step explanation:
Viral receptors are molecules found on the surface of host cells that viruses use to attach themselves as a first step in infection. The attachment via viral receptors is necessary for the virus to penetrate the cell membrane and replicate inside the host cell. Receptors have their own physiological functions; however, viruses have evolved to exploit them for entry into the cell. An example is the HIV virus which uses the CD4 molecule on T lymphocytes as a receptor.
The specificity of viral attachment means that viruses can often only infect particular species or cell types. For instance, the virus causing human influenza targets receptors specific to cells of the respiratory system, which helps explain the limited host range of many viruses.