When you have a cold virus in your body the virus attacks your bodies cells and that makes you feel weak and tired.
Then your immune system kicks in to help fight off the virus that is attacking your bodies cells and the immune system eventually kills off the cold virus within a week to 10 days and then you recover from the cold.
The cold virus usually gets into our bodies through our noses and then the cold virus hijacks the cell to help it make more viruses that go on to infect other cells, preparing to invade your nose and other cells.
There's no cure for the common cold though other than getting plenty of rest, taking some cold medicine getting Vitamin C, Staying Hydrated and using a humidifier.
Your body has a good way of naturally fighting off the cold virus one it attacks your body so that's why you feel fatigued during a cold virus.