Objection 1. It would seem that a person does not incur excommunication for communicating with one who is excommunicated. For a heathen or a Jew is more separated from the Church than a person who is excommunicated. But one does not incur excommunication for communicating with a heathen or a Jew. Neither, therefore, does one for communicating with an excommunicated Christian.
Objection 2. Further, if a man incurs excommunication for communicating with an excommunicated person, for the same reason a third would incur excommunication for communicating with him, and thus one might go on indefinitely, which would seem absurd. Therefore one does not incur excommunication for communicating with one who is excommunicated.
On the contrary, An excommunicated person is banished from communion. Therefore whoever communicates with him leaves the communion of the Church: and hence he seems to be excommunicated.
I answer that, A person may incur excommunication in two ways. First, so that the excommunication includes both himself and whosoever communicates with him: and then, without any doubt, whoever communicates with him, incurs a major excommunication. Secondly, so that the excommunication is simply pronounced on him; and then a man may communicate with him either in his crime, by counsel, help or favor, in which case again he incurs the major excommunication, or he may communicate with him in other things by speaking to him, greeting him, or eating with him, in which case he incurs the minor excommunication.
Reply to Objection 1. The Church has no intention of correcting unbelievers as well as the faithful who are under her care: hence she does not sever those, whom she excommunicates, from the fellowship of unbelievers, as she does from the communion of the faithful over whom she exercises a certain power.
Reply to Objection 2. It is lawful to hold communion with one who has incurred a minor excommunication, so that excommunication does not pass on to a third person.
Source: Summa Theologica (New Advent)