Is Herpes Zoster infectious ? ( Shingles )?

If you have never had chickenpox, avoid contact with people who have shingles or chickenpox. Fluid from shingles blisters is contagious, and exposure to it can cause chickenpox (but not shingles) in people who have never had chickenpox.

If you develop shingles, avoid close contact with people until after the rash blisters heal. It is especially important to avoid contact with people who are at special risk from chickenpox or shingles, such as:

Pregnant women, infants, children, or anyone who has never had chickenpox.
Anyone who is currently ill.
Anyone with a weak immune system who is unable to fight infection (such as someone with HIV infection or diabetes).
One study reports that the virus that causes shingles may be released into the air from shingles sores. If you have active shingles, you may be able to prevent spreading the disease by covering the shingles sores with a type of dressing that absorbs fluids and protects the sore (hydrocolloid dressing, such as DuoDerm). 4

Shingles is a mild form of chicken pox. People who have had Chicken Pox before and have lost part of their immunity to the Virus get Shingles instead of Chicken Pox.
So that said it is contagious to people who have not had Chicken Pox. i.e. they will get full blown Chicken Pox instead of just Shingles as if Shingles was not bad enough.
Same precautions apply as with Chicken Pox to keep from spreading it.