No doubt I am a bit paranoid but it has served me well. I just want to know what happens to any visa holder in a worst case scenario.
You do all that work to get there then find out you have a disease or sickness that is going to require weeks or months of treatment and you have leave school or you job. Are you totally out of luck? Would the NHS ship an ill person out for treatment in there own country?
The NHS wouldn't do anything - they are legally obliged to treat you if you need treatment.
The problem would come if you had to leave school or your job completely, because in that case your visa would most likely no longer be valid, which means you would no longer be legally allowed to live in the UK, you would not be allowed free access to the NHS (in that case you would have to pay for all treatment except ER emergency treatmenet) and you would have to leave.
With a work visa, your permission to live in the UK is based on you being employed by your UK company. If you were sick long-term, you may be entitled to sick pay provisions while still being employed by the company, but if you were forced to leave the job completely, you would have to leave the UK because your visa would not be valid anymore (and you would not be entitled to free NHS treatment without a valid visa anyway).
Similarly with a student visa, if there aren't any provisions for continuing to stay enrolled at the university while you are sick, and you had no choice but to drop out, you would have to leave the UK, otherwise you would become an illegal immigrant
So, it's about the validity of your visa, not about what the NHS would do.
Having said that though, is there any particular reason why you think you might get sick? Do you have underlying conditions that make you think you might be susceptible to bad illnesses?
Chances are that your friend with the auto-immune disease would have been diagnosed with it anyway, regardless of whether they were living in London or not. And anyone can respiratory infections anywhere - I doubt it had anything to do with London as a city.