8. A system, comprising: a data processing apparatus; and a memory storage apparatus in data communication with the data processing apparatus, the memory storage apparatus storing instructions executable by the data processing apparatus and that upon such execution cause the data processing apparatus to perform operations comprising: identifying, in a set of text sources that were previously published within a specified amount of time of a current time, two or more keywords that co-occur in at least one of the text sources; determining that the two or more identified keywords are not designated as co-occurring keywords in a data structure storing historically linked co-occurring keywords; in response to determining that the two or more identified keywords are not designated as co-occurring keywords and in response to the two or more identified keywords co-occurring in at least one of the text sources, temporarily linking the two or more identified keywords for a limited amount of time; and distributing, during the limited amount of time, content items based on the temporary link between the two or more identified keywords.
8. A system, comprising: a data processing apparatus; and a memory storage apparatus in data communication with the data processing apparatus, the memory storage apparatus storing instructions executable by the data processing apparatus and that upon such execution cause the data processing apparatus to perform operations comprising: identifying, in a set of text sources that were previously published within a specified amount of time of a current time, two or more keywords that co-occur in at least one of the text sources; determining that the two or more identified keywords are not designated as co-occurring keywords in a data structure storing historically linked co-occurring keywords; in response to determining that the two or more identified keywords are not designated as co-occurring keywords and in response to the two or more identified keywords co-occurring in at least one of the text sources, temporarily linking the two or more identified keywords for a limited amount of time; and distributing, during the limited amount of time, content items based on the temporary link between the two or more identified keywords. 13. The system of claim 8 , wherein the operations further comprise adjusting the limited amount of time based on a performance of content items distributed based on the temporary link between the two or more identified keywords.