Chickenpox - Causes of chickenpox
NHS Choices Medical Reference
Chickenpox is caused by the varicella-zoster virus. You catch it by coming into contact with someone who is infected with the virus.
It's a very contagious infection. About 90% of people who have not previously had chickenpox will become infected when they come into contact with the virus.
How you catch the virus
The chickenpox virus is spread in the same ways as colds as flu. It's contained in the millions of tiny droplets that come out of the nose and mouth when an infected person sneezes or coughs. You can then become infected with the virus by breathing in these droplets from the air.
You can also become infected by handling a surface or object that these droplets have landed on, then transferring the virus to yourself by touching your face.
It takes seven to 21 days for the symptoms of chickenpox to show after you have come into contact with the virus. 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 the 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.
Read more about shingles.
Now, turn to our information on how chickenpox is diagnosed.