SNOMED CT "Concepts" are representational units that categorize all the things that characterize healthcare processes and need to be recorded therein. In 2011, SNOMED CT included more than 311,000 concepts, which are uniquely identified by a concept ID, e.g. the concept 22298006 refers to "Myocardial infarction". All SNOMED CT concepts are organized into acyclic taxonomic (is-a) hierarchies; for example, "Viral pneumonia" IS-A "Infectious pneumonia" IS-A "Pneumonia" IS-A "Lung disease". Concepts may have multiple parents, for example "Infectious pneumonia" is also a child of "Infectious disease". The taxonomic structure allows data to be recorded and later accessed at different levels of aggregation.