Dipiti, according to its home page, indexes over 2300 communities and almost 29 million conversations. They’re divided broadly into health, legal, money, and pet care. (So this is not the search engine to find hockey conversations or discussions of politics.) There’s a tag cloud of topics or you can search within one of the aforementioned categories (or all of them if you like.)
I did a search for psoriasis (long story) and got 11 results. The results were for sites, and they included Topix.com, CureSearch, ButYouDontLookSick.com, and Self. Each listing contained a brief excerpt and a count of related conversations (from 1 to over 50.) A couple of the communities had user ratings while they all had a Dipiti score, which grades the sites based on a number of factors (explanation here.) If you want to pursue the related conversations, it looks like you go straight to the site.
I’ll try this search engine the next time I need to do some health searches, but I wish it offered some kind of RSS feed and that there was an easier way to go straight to relevant conversations.