Shingles (herpes zoster) is an infection caused by varicella-zoster virus (VZV), the virus responsible for chicken pox.  After a person recovers from chickenpox, VZV travels from the skin along the nerves and into a section of the nerves located near the spinal cord (dorsal root ganglia). The virus may lie dormant in the dorsal root ganglia for decades. However, in some people it reactivates.

Why VZV becomes active after a prolonged period of dormancy (inactivity) is unknown. Research suggests that several factors may play a role in who develops shingles. These factors include:

• A weakened immune system due to age, organ transplantation, disease (e.g. HIV/AIDS) or treatment (e.g., radiation, medication)
  
  
• Stress
  
  
• Fatigue
  
  
• Injury to the area of the skin where the rash subsequently occurs

Once reactivation of the virus is initiated it begins to multiply within the dorsal root ganglia. The damage this causes within the nerve triggers pain. The virus then makes its way back along the nerve to the skin, where it presents as a rash.

Shingles is only contagious from direct contact with open blisters. Individuals who have not had chickenpox or received the vaccine and are then exposed to shingles are not at risk of developing shingles, but may develop chickenpox. Covering open blisters may be an effective way of preventing the spread of VZV to others. Likewise, scratching of blisters may worsen symptoms or spread VZV to others and should be avoided.