Over the winter, every other advert tells us that we need to boost our immune systems. If we get a cold, it's proof that our immune system must be failing. Two and it's not normal. But how do you know what's normal? The immune system is incredibly complex and no two people are the same - even siblings. Factors like aging - the older you are, the weaker your immune system is likely to be; stress; exposure to microbes - yes, being around dirt!; and genetics all play a part. Vivienne Parry does her best to find out what's normal.