This coat is called a capsid. Some virons contain an envelope, constructed from the host cell membrane. The shape of these virons vary from spherical, cylindrical, bullet-shaped , to amorphous shaped particles. They vary in diameter from 18 to 300 nm. Virons must infect a living cell.
Viruses are obligate intracellular parasites.
These are called retro viruses. HIV is an eample of a retro virus. They are guided by external recognition areas of the host cell. This may keep a virus from infecting many different types of cells. The phage lands on the host cell, and using 140 ATP's in its tail fibers, injects its nucleic acid core into the bacterium.