1. A method performed by data processing apparatus, the method comprising: determining a first frequency of occurrence of a phrase in a plurality of first text items describing business entities, wherein the first text items are received from a trusted source; determining a first ratio of the first frequency of occurrence to a count of the plurality of first text items by dividing the first frequency by the count of the plurality of first items; determining a second frequency of occurrence of the phrase in a plurality of second text items describing business entities, wherein the second text items are received from an untrusted source; determining a second ratio of the second frequency of occurrence to a count of the plurality of second text items by dividing the second frequency by the count of the plurality of second items; determining a likelihood that text items received from the untrusted source include spam; determining a likelihood that the phrase is spam based at least partially on the first ratio, the second ratio, and the likelihood that text items received from the untrusted source include spam; and determining a likelihood that a different text item received from the untrusted source and that includes the phrase is spam based at least in part on the likelihood that the phrase is spam.