So the thing is...and this is what most people struggle to get their head around...is that this is a NEW virus. So unlike me most viruses out there, virtually no one has any immunity.
This means that, because it is actually quite infectious, if you are exposed to it at all, you are likely to catch it.
For MOST people this will be a mild illness, bit because SO MANY are at risk of contracting it, the ACTUAL numbers of serious cases are HUGE.
And those serious cases, despite being a small proportion of TOTAL cases are enough to overwhelm and collapse the whole heath care system. (this is not unique to or a failure of the NHS but a function of the pandemic)