I agree, Hester - while there are some aspects of EBD's depictions of illness that reflect pre-antibiotic treatment methods, a lot of it doesn't reflect any medical reality, and is more about her heroines' literary descent from fragile, swooning Victorian heroines, but updated to the school story era, so now they have to be 'jolly' as well.
Robin is a kind of non-dying combination of Little Nell and Little Women's Beth, and Joey faints as frequently as Gothic novel heroines - including one adult 'faint' where, having seen her son climb over a cliff to get at a bird's nest, she swoons and apparently stays unconscious for two hours, which would presumably merit an MRI these days at the very least.
One of the medical implausibilities that struck me was that Joyce and Gillian Linton's mother discovered she had advanced TB and was forbidden from attending public gatherings in the UK by her doctor, presumably for fear of her infecting others. She then travels to the San, becoming seriously ill on the way, with her two daughters, who are presumably at some risk of infection by proximity, and what's Joey's response? Annoyed that Joyce, potentially pullulating with TB, didn't give the delicate and TB-prone Robin a good-night kiss on her first night!
(Though as we have established, Robin is a cyborg 'trained to instant obedience', so not in fact at risk.)