Shingles Support Group
Herpes zoster, colloquially known as shingles, is the reactivation of varicella zoster virus, leading to a crop of painful blisters over the area of a dermatome. It occurs very rarely in children and adults, but its incidence is high in the elderly (over 60), as well as in any age group of immunocompromised patients.

"Shingles cannot be passed from one person to another. However, the virus that causes shingles, the varicella zoster virus, can be spread from a person with active shingles to a person who has never had chickenpox. In such cases, the person exposed to the virus might develop chickenpox, but they would not develop shingles. The virus is spread through direct contact with fluid from the rash blisters, not through sneezing, coughing or casual contact.
A person with shingles can spread the virus when the rash is in the blister-phase. A person is not infectious before blisters appear. Once the rash has developed crusts, the person is no longer contagious.
Shingles is less contagious than chickenpox and the risk of a person with shingles spreading the virus is low if the rash is covered."
http://www.cdc.gov/shingles/about/transmission.html
I'm not sure how far to believe this, "low" is not "zero," and in my opinion, it's best to avoid contact with any virus-related rash.
I really don't think the medical community is 100% sure what will cause the virus to become active, but the things I mentioned seem to be present in those who have a shingles outbreak. Not all of them in one person, but generally one of the stressors can be found.
You can't 'give' shingles to others, however, if you are around someone who hasn't had chicken pox and they come in contact with the fluid from the blisters, they can get chicken pox.
This and all other aspects of this miserable condition are covered in my ebook, Understanding Shingles.
http://theunderstandingseries.com/book-review/understanding-shingles/