Shingles and autoimmune diseases?

So, I am starting to get the shingles virus for the second time in a year. This time I got to the doctor before a scar has appeared and he has given me the anti-viral Valtrex, which can be used to treat shingles. I suspect the fact that I am susceptible to shingles may have something to do with the fact that I also have psoriasis, which is an autoimmune disease. My Dad also has psoriasis and he even got the shingles at the same age. Does anyone know more information about this? I've looked at a couple of websites but I could use a more expert opinion from someone who may have had experience dealing with or researching this.