Chickenpox is caused by the varicella virus, which grows quickly in cells lining the back of the throat, lungs and upper part of the respiratory tract.
‘It then takes about two weeks to spread by piggybacking on white blood cells as they travel around the body,’ says Dr Ben Neuman, a microbiologist from the University of Reading.
‘The virus infects skin cells and the nerve cells under-neath them.’
Once the virus has taken hold, a rash and then blisters form in about two days. You will be infectious until the blisters dry out — about a week — and are covered in scabs.
However, some of the virus remains in nerve cells at the bottom of the spinal cord, where it can be reactivated as shingles years later when the immunity is weakened.