In Britain, more than 700,000 people have dementia, a figure that is estimated to rise to more than a million by 2025. The Association of British Insurers' (ABI) now lists Alzheimer's as one of the most common critical illnesses that should be covered. Insurers must use this list. Kevin Carr, head of protection strategy at LifeSearch claims that you will be considered at higher risk if one of your parents suffers from the disease but that doesn't necessarily mean you will be refused cover altogether.

'One of the main considerations is not only what your parents suffered from, but how old they were when they were diagnosed. If your parent contracted a disease in their forties, and when you take your policy out you are also in your forties, then your insurer would see you as more of a risk. But in general, the individual circumstances of your own health will dictate whether or not a family history of any specific illness will have an effect on the cost of your premium.

Ultimately, if you have a partner and dependants and a fairly large amount of debt in the form of a mortgage, then you need to give some thought to what might happen and what the potential cost of losing a salary might be. All critical illness policies have to cover 23 core conditions which are recommended by the ABI. This includes seven of the most likely illnesses or treatments (certain types of cancer, kidney failure, major organ transplants, open heart surgery, heart attacks, strokes and multiple sclerosis). Any other diseases will be defined by the insurer.

Moneysupermarket's head of protection, Emma Walker recommends insurers such as Legal & General and Axa because their cover extends further than the ABI imposes (they each cover more than 30 conditions).

Bupa covers about 40 illnesses, yet clearly states which non-ABI defined conditions it will include (for example, it will only cover insulin-dependent diabetes if diagnosed above the age of 45). Walker says it is a good policy if you're seeking extra benefits like as advice and counselling on keeping yourself healthy. Provider Bright Grey offers a 'Helping Hand' service, which gives therapists, specialist nurses and family support to all its policyholders

Prudential offers' serious illness' cover, which gives small payouts for minor conditions that are not usually covered on other critical illness policies. It plans to launch a new product next week which it assures will 'revolutionise the critical illness market'.

Most banks, including Nationwide and Halifax, offer critical illness cover when you take out a mortgage, but Lifesearch advises customers to take cover only if they fully understand what the policy does and doesn't do. If the cover does turn out to be wrong for you, you might not be able to complain if you bought a product directly from your lender.