code: def get_all_unique_diseases(self):
          
          all_disease_ids = self.graph.vs["associated_diseases"]
          # remove None values from list
          all_disease_ids = [lst for lst in all_disease_ids if lst is not None]
          # flatten list of lists, get unique elements
          all_disease_ids = list(set([id for sublist in all_disease_ids for id in sublist]))
          return all_disease_ids
docs: Get all unique diseases that are known to the network.
  
          :return: All unique disease identifiers.