Final answer:
Viruses infect cells by specifically binding to receptors on the host cell, entering the cell, and hijacking the cell's machinery to replicate its genetic material and synthesize proteins. New viruses are then assembled and released, often resulting in the death of the host cell.
Step-by-step explanation:
Viruses are obligate, intracellular parasites that require a host cell to replicate. To infect a cell, a virus attaches to a specific receptor on the host-cell membrane. This specificity comes from surface molecules known as viral receptors. Once attached, the virus is taken into the cell, either by penetration or endocytosis, and the viral genetic material is released inside the host cell.
The virus then utilizes the host cell's machinery to replicate its genome and synthesize viral proteins. After replication, new virions are assembled and released from the host cell, which is often damaged or destroyed in the process, through lysis or apoptosis. This process can cause various symptoms of viral diseases in the host organism, which are a combination of cellular damage and the host's immune response trying to eliminate the infection.
Viral specificity to host cells is crucial because a virus can only infect and replicate within cells that have the correct receptor molecules that they can bind to. The host's metabolic machinery and expression of certain genes also play significant roles in whether a virus can successfully replicate within that cell.