In chapter VIII of Angela's Ashes Frank is about to celebrate his confirmation, which is a huge religious moment in the Catholic Faith. It entails that you will take the oath to remain a Catholic, and die a Catholic. This is a highly awaited moment by the McCourt family. However, as with many instances in their lives, another obstacle will meet the McCourts halfway and will prevent this moment of joy from happening.
This time, it was disease. After a nosebleed that would not stop Frank is diagnosed with typhoid. Being that the McCourts were a big family that had already lost children to disease, the thought of keeping Frank around while being contagious was out of the question. Therefore, when he was moved to the typhoid ward and no visitors were allowed, Frank parents had no issue and quit visiting altogether. It was not so much a choice as it was a necessary precaution.
This is not to say that Frank was hurt, or unhappy, to be by himself. He actually found some solace in the ward and read books. He was even scolded by speaking to the kids diagnosed with diphtheria.
Typhoid doesn't talk to Diphtheria
Frank returned home weeks later, to the sad news that he had to make up for the academic time he lost from school. Out of the entire situation, this latter fact was the only thing Frank truly felt upset about.
Frank's disease is contagious, so his family has no choice but to stop visiting him.