Modern lifestyles have increased the external threats to our
health. These include carcinogens from pollution (obviously smokers
are at major risk), bacteria and viruses which attack the immune
system (of which HIV is one of the most virulent) and a growing
use of immuno-suppressant medication.
At the same time, levels of micro-nutrients in the diet, vital
for immune function, have become reduced.
If the level of threat increases and most defences weaken, the
chances of illness must increase.
Nevertheless, disease only occurs when your immune system is
finally overwhelmed by the attacking organisms.
For example, it is thought that cancers start to grow relatively
frequently in our bodies, but most don't become a problem because
the immune system spots that the cells are different - and dispatches
'killer cells' to destroy them.
The immune system is highly complex and depends on the normal
functioning of a large number of immune cells.
Indeed there are millions of specialist cells circulating that
spot different types of invader, and dispose of them.