In the book Angela's Ashes, why did Sister Rita forbid Frank to talk to Patricia?

The hospital has very strict rules, not all of which make a lot of sense. The Catholic Church is a primary influence in Ireland at this time, and part of the reason Sister Rita has forbidden Frank to talk to Patricia is because "there's to be no talking between two rooms especially when it's a boy and a girl." The thought behind this bann is that by forbidding boys and girls to fraternize, issues of sexual immorality might be avoided. The rule is rigidly enforced; it does not matter that the two individuals involved are just children, and that they are only talking to pass the time, sharing poetry.

Frankie has typhoid, and is in a room for typhoid patients, while Patricia has "diphtheria and something else," and is in a room for diphtheria patients. According to Seamus, a custodian who sympathizes with the two children's desire for communication,

"I'm not supposed to be bringing anything from a dipteria room to a typhoid room with all the germs flying around and hiding between the pages and if you ever catch dipteria on top of the typhoid they'll know and I'll lose my good job and be out on the street."

According to hospital rules, items cannot be exchanged between rooms housing patients with different diseases because of concerns that germs might be transmitted from one room to another. Neither, apparently, is talking between the rooms allowed. The nurse shouts at Frankie and Patricia, "no talking between rooms. Diptheria is never allowed to talk to typhoid and visa versa," while Sister Rita herself reinforces the decree, reminding the children of the "ban on all talk between typhoid and diphtheria" (Chapter 8).