Symptoms appear 10 days to 3 weeks after exposure to someone else with the disease, usually an infected child. People usually feel generally unwell and have one to two days of fever, although young children do not always get these early symptoms.
A rash of red, itchy spots then starts to appear, usually on the face and scalp at first, about 24 hours after the onset of the fever. The spots turn into blisters filled with fluid, and may spread to other parts of the body including the chest, stomach, arms, legs and underarms. Some people have just a few spots, but in others they can cover the whole body.
Occasionally the blisters may become infected with bacteria from the skin and need treatment with antibiotics. In rare cases these infections can be more serious and require hospital treatment. They can include severe skin infections, lymph node infection, septicaemia or blood poisoning, necrotising fasciitis and toxic shock syndrome.
Rarely, chickenpox can cause more serious symptoms, for example a form of life-threatening pneumonia (lung infection). It is important to contact a doctor immediately if anyone with chickenpox develops pain in the chest or has difficulty breathing.
Two other rare but serious complications are chickenpox meningitis and inflammation in the brain (encephalitis and cerebellar ataxia). Serious complications occur most often in certain high-risk groups. See 'More information about the vaccine' below.
The virus that causes chickenpox stays in our bodies throughout our lives and can go on to cause shingles, usually many years later.