Wikipedia is likely referring to examples like the one in [the introduction][1] to Koller and Friedman's book.  In such examples one forms a directed graphical model with variables for underlying causes of diseases, variables for diseases themselves, and variables for symptoms.  These are connected based on which causes could lead to which diseases and which diseases lead to which symptoms, using known data on the conditional probabilities involved.  One then takes symptoms as given and, conditioned on these, infers the states of the hidden disease variables.


  [1]: http://mitpress.mit.edu/books/chapters/0262013193chap1.pdf