Acne on Scalp, Face, and Back

Approx. 6 weeks ago, I started to develop acne on my face and scalp.  The last week I've gotten three acne bumps on my back.  Throughout the day, I feel a tingling on my scalp.  My hair seems to be growing at a slower pace.
My concern is that this could have been caused by a encounter with a commercial sex worker about 5 months ago.  I have taken the majority of STD tests since than and all have come back negative.  The only test I haven't taken is herpes.  I find it too coincidental that this has happened to me after my sexual encounter.  The last time I had acne issues was when I was a teenager about 15 years ago and I never had acne on my scalp.

Here's my questions:

1)  Can an STD cause acne?

2)  What are the all the causes of adult acne?  What is the best and worst case scenario's?

3)  Is it possible that any scars I get from the acne could never go away?

[chellybeans]

1. not usually
2. hormones, bacterial infection, overactive sebaceous glands, rosacea, allergies etc
3. yes