Chickenpox is caused by the varicella-zoster virus. You catch it by coming into contact with someone who is infected.
Chickenpox is a very contagious infection. Around 90% of people who have not previously had chickenpox will become infected when they come into contact with the virus.
The chickenpox virus is spread most easily from someone who has the rash. The blisters are very itchy and break open easily, which can contaminate surfaces or objects. The virus may then be transferred by touching the surface or object, then touching your face.
The virus is also contained in the millions of tiny droplets that come out of the nose and mouth when an infected person coughs or sneezes. This can also contaminate surfaces or objects.
It normally takes 14 days for the Symptoms of Chickenpox to show after you have come into contact with the virus. However, this can vary from person to person – from as little as 7 days, up to 21 days. This is called the “incubation period”.
Someone with chickenpox is most infectious from one to two days before the rash appears, until all the blisters have crusted over. This usually takes five to six days from the start of the rash. Read More About: Diagnosis of Chickenpox and how to stop chickenpox spreading.
If you have not had chickenpox before, you can also catch chickenpox from someone with shingles (an infection caused by the same virus). However, it’s not possible to catch shingles from someone who has chickenpox.
SOURCE: NHS UK