15. A non-transitory computer storage medium encoded with a computer program, the program comprising instructions that when executed by a data processing apparatus 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.
15. A non-transitory computer storage medium encoded with a computer program, the program comprising instructions that when executed by a data processing apparatus 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. 16. The non-transitory computer storage medium of claim 15 , wherein distributing content items based on the temporary link between the two or more identified keywords comprises: receiving, during the limited amount of time, a content item that specifies a first keyword of the two or more identified keywords; selecting, based on the temporary link, a content item related to a second keyword of the two or more identified keywords, the second keyword being different from the first keyword; and transmitting, in response to the content item request, the selected content item.