<P> A third and more specific example, is by simply attaching to the surface of the cell via receptors on the cell, and injecting only its genome into the cell, leaving the rest of the virus on the surface . This is restricted to viruses in which only the gene is required for infection of a cell (most positive - sense, single - stranded RNA viruses because they can be immediately translated) and further restricted to viruses that actually exhibit this behavior . The best studied example includes the bacteriophages; for example, when the tail fibers of the T2 phage land on a cell, its central sheath pierces the cell membrane and the phage injects DNA from the head capsid directly into the cell . </P> <P> Once a virus is in a cell, it will activate formation of proteins (either by itself or using the host) to gain full control of the host cell, if it is able to . Control mechanisms include the suppression of intrinsic cell defenses, suppression of cell signaling and suppression of host cellular transcription and translation . Often, it is these cytotoxic effects that lead to the death and decline of a cell infected by a virus . </P> <P> A cell is classified as susceptible to a virus if the virus is able to enter the cell . After the introduction of the viral particle, unpacking of the contents (viral proteins in the tegument and the viral genome via some form of nucleic acid) occurs as preparation of the next stage of viral infection: viral replication . </P>

Virus tail fibers are used for attachment to the host cell