It is a disease caused by a virus that mostly affects children. The virus spreads easily and is very contagious. People of all ages who get chickenpox can get very sick, especially people with weakened immune systems. A fever, headache, feeling unwell, and loss of appetite start 2 to 3 weeks after contact with an infected person. These symptoms last for about 1 week and are followed by a rash that turns into blisters all over the body. Complications, such as infections of the lungs, skin, or brain, can occur. After recovery, the virus remains in the body and can cause a painful illness, shingles, later in life.
The virus is spread through the air (when you are in the same room as someone with chickenpox) or by direct contact with the rash. Chickenpox can spread 2 days before the rash starts and until the blisters have all turned to scabies. A pregnant woman who has contracted chickenpox can pass the disease on to her baby. If you or your child have not been vaccinated and have never had chickenpox, you are not protected against the virus and you are at risk of getting chickenpox and spreading it.
Source: Health Canada