You body is constantly being put under attack by microscopic bugs and virus trying to make you ill. The reason why we are not constantly ill is thanks to our immune system that seeks out and destroys these viruses and diseases before they can do any damage to our cells.
Cold symptoms, tiredness and lack of energy are the effects of our body's weakening as our body put's all its effort into trying to destroy the invading bacteria or viruses. Our body's are very clever and will adapt to try to fight off or prevent further diseases and bacteria entering our body's...If you get a runny nose for instance this is a attempt to stop further bad bacteria and viruses from entering your body.
However just like all things the immune system can weaken if we don't look after it. Sometimes it can be from things out of our control... white blood cell's (the cells that seek out and destroy invading body's in your blood) communication with other white blood cells can become distorted by magnetic radio waves.. that can be caused by the sun.
Eating the wrong food can also cause white blood cells to not work as efficiently So eating a good diet is very important and one of the ways you can prevent yourself becoming ill.