Final answer:
Viruses invade host cells by attaching to specific receptors on the surface of those cells, with each virus requiring a particular receptor, similar to a key fitting a specific lock.
Step-by-step explanation:
A virus invades a host cell based on the receptors on the outer surface of the host cells. Viruses use these cell-surface molecular structures for recognition and attachment, which is crucial for viral entry and subsequent infection. For example, the human influenza virus specifically binds to receptors on the membranes of respiratory system cells. Viruses such as HIV utilize the CD4 molecule on T lymphocytes as a receptor, highlighting the specificity in the virus-host interaction. It is evident that this specificity of viral attachment to host receptors is akin to a key fitting into a lock, ensuring that only certain viruses can infect specific cell types within particular species.