Virus particles contain the viral genome packaged in a protein coat called the capsid. For some viruses, the capsid is surrounded by lipid bilayer that contains viral proteins, usually including the proteins that enable the virus to bind to the host cells. This lipid and protein structure is called the virus envelope, and is derived from the host cell membranes. The capsid and envelope play many roles in viral infection, including virus attachment to cells, entry into cells, release of the capsid contents into the cells, and packaging of newly formed viral particles. The capsid and envelope are also responsible for transfer of the viral genetic material from one cell to another. These structures also determine the stability characteristics of the virus particle, such as resistance to chemical or physical inactivation.

Key concepts:

• Viral genetic material is packaged inside protein structures called capsids.
  
• Viruses are divided into two groups: enveloped viruses are surrounded by an outer lipid membrane; nonenveloped viruses lack this membrane.
  
• Where present, the envelope contains the viral proteins, which mediate binding to host cells. Where no envelope is present, this function is carried out by the outer capsid proteins. In general, nonenveloped viruses are more stable and can survive much longer in the environment.
  
• Capsids and envelopes determine the method of viral entry into and exit from host cells.