6. A method for conducting a search using an ontology-based search database, the method comprising: via at least one processor: performing the following using a search database that includes a first set of nodes corresponding to potential search results, a second set of nodes corresponding to entity-selectable search queries that are also terms of an ontology describing all of the potential search results, wherein each term is an attribute of a corresponding potential search result, and a third set of nodes corresponding to non-selectable terms that represent hidden states of the second set of nodes, wherein the nodes of the search database are connected in accordance with semantic relationships specified by the ontology and activated in response to search queries: obtaining, from a user, a search query matching one of the entity-selectable search queries; changing, in response to obtaining the query, a state of the node corresponding to the matching search query to an activated state; propagating the change in state of the node to at least one other node in the network, wherein the at least one other node includes a node in the third set of nodes, and the propagating is performed: deterministically based on the semantic relationships, and probabilistically based on at least one of (ii) frequency information indicating a frequency with which a term of a node in the third set of nodes is known to occur when an associated item of a node in the first set of nodes also occurs and (ii) a predefined false positive or false negative rate; after the propagating, calculating a marginal probability value for each node of the first set of nodes based on the states of the third set of nodes; and outputting a search result ranking, ordered according to the calculated marginal probability values.
18. The method of claim 6 , wherein: the database includes items obtained from a medical information source; the potential search results correspond to potential medical conditions; and frequency information corresponding to a recorded occurrence of clinical abnormalities for the potential medical conditions is used to adjust the calculating of the marginal probability values.