Have you heard the one heard the quote about " empty barrels" ? the more you post the more you show how little you understand . If someone is "showing signs" of heart problems then they should stay away as they are in a high risk group and the drugs they take for some cancers lower your resistance to infection so they again should stay clear .
As said before it is not a black or white there is a middle ground ,you can have treatment without putting yourself at risk.
Yes chemotherapy does lower your white blood cell levels and you do have to be massively careful to avoid infection.
The problem here though is quite different, patients are missing GP referrals to hospitals for suspect cancerous conditions, in other words prior to biopsy and diagnosis.
They're presenting, if at all, at a much more advanced stage when its far harder, if possible at all, to treat successfully.
Presenting when the cancer has metastasised to other organs is almost always fatal. Cancer needs to be caught in the early stages, then it can be very treatable.
Any GP referrals to hospital should absolutely be attended, not doing so is risking it being too late and could be fatal.
Cancer is treated at specialist cancer units within hospitals, such as Clatterbridge or in the last decade Fazakerley - although others like Southport now also have centres. Clatterbridge was always the regional centre until relatively recently.
Chemotherapy and Radiotherapy are the usual methods of treatment with the former lowering your white blood cell count (gives a measure of your immune system and when very low you are very vulnerable)