Nobody is immune to this virus, because it is a new virus. From what I understand it is highly contagious, and spread by people sneezing and coughing, (or breathing?) and the droplets remain in the air - and active - for some time before settling on surfaces, when they can remain 'active' for several hours or days.
Other people get it, by breathing in the droplets (those still airborne), and by touching surfaces, which still contain an active virus, and then touching your face (ears, nose, eyes).
Therefore minimising distance (contact), will reduce risk of directly breathing in, and washing your hands frequently, and trying to avoid touching your face, will reduce the rate of infection (the speed by which it is spread).
Ultimately, we will probably all get it, unless it is entirely suppressed.
The herd immunity theory (not said by Boris, but by one of the scientists), is that, as no one is immune, then it will infect about 60 percent of the population before it will die out. ie then the virus will have difficulty being passed on, because most people will have had it.
The aim of the government is to avoid everyone getting it at once, and swamping the NHS, who will have to restrict treatment. More people would die.
If they 'flatten the curve', you may still get it, but hopefully resources will be sufficient that if required, then ventilators will be available.
My personal aim, is to
a) try to avoid getting it, just in case I am one of the unlucky ones who require hospital treatment, and
b) if I eventually DO get it, then hopefully I will have delayed infection until such a time as the NHS can cope better, and that I will not be one of those deemed too old to treat.