Although yeast infections are frequently associated with sexual activity, similar to the way that bacterial vaginosis is, they are not generally considered to be sexually transmitted diseases. It is possible for a candida (yeast) infection to be transmitted during sex, but that is a relatively rare occurrence. Far more common are yeast infections due to disruption in the normal vaginal flora or other non-sexual causes.
Many women find that they are particularly likely to experience yeast infections after finishing a course of antibiotics. This is because antibiotics can disrupt the lactobacilli that are the main component of the normal vaginal flora. These bacteria, cousins to the ones found in yogurt, produce several substances that inhibit the growth of yeast, and therefore a healthy vaginal ecosystem is considered somewhat protective against yeast infections.