A virus may also be covered by an envelope composed of lipids,proteins and carbohydrates. The envelope may also have spikes projecting from the surface. Whether a virus has an envelope or spikes is again determined by the nucleic acid. A virus can only infect and reproduce within certain host cells. Viruses identify their hosts by specific receptor molecules on the outside of the host cell.
Some viruses have broad host ranges and can infect many different types of cells.
Other viruses, on the other hand, have extremely narrow ranges and can infect only very specific cells. There are many variations of this cycle depending on the type of virus and the host. We will now examine bacteriophage lifecycle. The phage T4 and the phage lambda, for example, both infect E. coli. Phages, like all viruses, are obligatory intracellular parasites and must invade a host cell in order to reproduce.