Shingles is caused by the reactivation of the varicella-zoster virus, which is the virus that causes chickenpox.

After you have had chickenpox, the varicella-zoster virus lies dormant (inactive) inside your body. It can become reactivated at a later stage and cause shingles.

It is not known exactly why the virus is reactivated, but it is linked to having lowered immunity (protection against infection and diseases).

Your immunity to illness and infection can become lowered if there is a problem with your immune system (the body's natural defence system). This can happen as a result of:

• old age – as you age, your immunity may decrease, and shingles most commonly occurs in people over 70 years old
• physical and emotional stress – the chemicals released by your body when you are stressed can prevent your immune system working properly
• HIV and AIDS – people with HIV are much more likely to get shingles than the rest of the population because their immune system is weak
• recently having a bone marrow transplant – the conditioning you require before the transplant will weaken your immune system
• recently having an organ transplant – you may need to take medication to suppress your immune system so your body accepts the donated organ
• chemotherapy – chemotherapy medication, often used to treat cancer, can temporarily weaken your immune system

However, young people who appear otherwise healthy can also sometimes develop shingles.

Is shingles contagious?

It is not possible to catch shingles from someone else with the condition, or from someone with chickenpox.

However, it is possible for someone who has never had chickenpox to catch it from someone with shingles, as the shingles blisters contains the live virus.

In the UK, chickenpox is so common during childhood that 9 out of 10 adults have already had it and will not be at risk from someone with shingles.

Catching chickenpox

The blisters that develop as a result of shingles contain virus particles. If you have not had chickenpox before, you can catch it from direct contact with the fluid from the blisters of someone who has shingles, or from something that has the fluid on it, such as bed sheets or a towel.

If you have shingles, you are contagious until the last blister has scabbed over. This will usually occur after about 10 to 14 days.

Preventing the virus spreading

If you have the shingles rash, do not share towels or flannels, go swimming, or play contact sports. This will help prevent the virus being passed on to someone who has not had chickenpox.

You should also avoid work or school if your rash is weeping (oozing fluid) and cannot be covered.

Chickenpox can be particularly dangerous for certain groups of people. If you have shingles, avoid:

• women who are pregnant and have not had chickenpox before as they could catch it from you, which may harm their unborn baby
• people who have a weak immune system, such as someone with HIV or AIDS
• babies less than one month old, unless it is your own baby, in which case your baby should have antibodies (proteins that fight infection) to protect them from the virus

Once your blisters have dried and scabbed over, you are no longer contagious and will not need to avoid anyone.