I was/am underweight also. I suffered from Candida and did juice fasting, colonics, Liver Flushes and completely changed my diet. The really sucky part about all of it was that I ended up losing more weight! I recently finished a round of Humaworm (30 day parasite cleanse) and last night my husband told me I looked like I'd gained a little weight. I noticed I'd been gaining weight, but I didn't know it was obvious to anyone else. In my case, the things that I can directly attribute to gaining weight and healing my intestines are:
1. Humaworm cleanse (most of the time parasites and Candida go hand in hand)
2. Moreless' alkalizing drink
Since I'm done with the Humaworm cleanse (passed a lot of different weird parasites), I'm supplementing with some good probiotics (Primal Defense), continuing with the Alkalizing drink, taking small doses of high quality cod liver oil (I'm still having a hard time digesting fats, but I believe this will subside once I build up my mineral reserves) and continuing to eat right. Anyway, I hope you'll at least look into those forums to do a little research and draw your own conclusions. The names of the forums are AskHumaworm and AskMoreless: pH.