VIRUSES
Viruses are obligate, intracellular parasites that must enter another cell in order to multiply. Viruses are made up of genetic material surrounded by a protein shell called a capsid. The capsid protects the genetic material from degradation by exonucleases and, for some viruses, plays a role in the attachment of the virus to target cells. The proteins that form the capsid are known as protomers and these join together in a process known as self-assembly. As the protomers assemble, the characteristic helical or icosahedral shape of the capsid takes form. Some viruses have capsids that are surrounded by portions of the host cell's membrane; these types of viruses are referred to as "enveloped."

Some viruses have spike proteins that extend from the capsid, whereas enveloped viruses have spike proteins that project from the envelope. These spike proteins are important in recognizing and binding to specific structures on the surface of target cells. This is an important feature because the spike proteins define the types of cells that the virus is able to attach to and enter. Antibodies, protective molecules produced by the immune system, are able to bind to the spike proteins and prevent viral attachment to target cells.