Airlines have the right to refuse travel to anyone who is unwell, or who its agents suspect may be contagious. Sufferers who’ve passed the contagious stage, but are still showing signs of ‘infection’ such as spots, should carry a letter from their doctor confirming they are no longer at risk of spreading the disease. Typically, if you or your child has chickenpox, you could be barred from boarding unless all your blisters have turned to scabs. However guidelines differ widely across major airlines.