Introduction to Viruses
As we have already discussed, viruses are not considered to be alive. They cannot, on their own, perform many of the processes that are characteristic of life, including reproduction. Viruses absolutely require a host cell to propagate.
All viruses have the same basic structure. They are comprised of a protein coat surrounding the genetic material (which can be either DNA or RNA). In some viruses, an outer envelope is present, comprised of lipids and proteins derived from the cell membrane of their host.
Viruses come in a wide variety of shapes and sizes and have great capacity to mutate, especially in their protein coat and outer envelope. This often makes it difficult for the host organism to mount an immune response.