You are covered in (and full of) fungus...

Whilst it may sound a bit unpleasant, almost from the moment we are
born, every person on the face of the earth is living in their own
personal primordial soup of micro-organisms such as bacteria, fungi and
yeast. These microbes live on your skin, in your mouth, nose and
digestive tract so that every part of you both externally and internally is
awash with microbes from the moment you are born until you take your
very last breath.
It is estimated that among these micro-organisms that live on and in
you throughout your life, there are several hundred different types of
yeasts. Fortunately, only a very small handful of these yeast cultures
are potentially harmful, with the primary cause of yeast infections being
one particular strain known as Candida albicans.


This is a yeast that lives in the mouth, throat, nose, intestines and all
over the skin of most normal human being. It is also considered to be a
normal part of the internal lining of the bowel, and in its normal state
actually helps to ensure that regular, normal bowel movements are
maintained.
Candida albicans first attaches itself to newborn babies either during or
very shortly after birth, but it remains essentially harmless even to a
baby as long as that baby has an immune system that is strong enough
to keep the growth of the yeast culture in check.
By the age of six months, around 90% of babies will test positive for
the presence of Candida albicans. This is why babies whose immune
systems are somehow weakened are often prone to oral thrush in the
first few months of their life, as this is a yeast infection caused by
Candida.
By the time we are adults, almost every person plays host to Candida
albicans. However, for the vast majority of the time, this is not going to
cause any kind of problem at all, any more than any other of the
millions of microbes on your body will do.
This is because for most of the time, there is a balance between all of
these different strains of microbes, and as long as this balance is
maintained, you will remain healthy and infection free. For example,
there are other bacteria on your skin that fight against Candida albicans
for the same food sources, and consequently this keeps the Candida cell
growth in check.
It is only when this balance is somehow upset that the conditions will
prevail whereby a yeast infection can begin to set in.