Chickenpox is caused by the varicella zoster virus and is very contagious. If you've never had it and you live someone who has it, you have a very high chance of catching it. The virus spreads through the air in the fine spray of saliva or mucous droplets when you cough or sneeze. It can also be passed on through contact with the fluid from chickenpox blisters. If you have chickenpox, you can pass it on from about 1 to 2days before the rash breaks out(the most infectious period) until your spots have crusted over, usually 5-6days after the illness starts. Children should be kept home from school or nursery during this time. Once you have had chickenpox, you will usually be immune and won't catch it again. However, on rare occasions people can get it again.
Attribution: BUPA