What will the NHS look like in the future? All the main parties are committed to an NHS free at the point of delivery, but whether it can continue to provide comprehensive care is up for debate.
Health and care are devolved powers. Westminster MPs only have legislative powers and responsibilities for health in England.
What's at stake?
The problem facing the NHS is clear. Over the past 60 years the focus has shifted from curing illness, such as infectious diseases, to managing long-term illnesses, like dementia, for which there are no cures.
That is far more expensive and is putting a strain on services. Health is actually a devolved issue and while the pressures are being felt most acutely in England, they are present across the whole of the UK.
While the health service has escaped the budget cuts foisted on most other public services - funding has risen by 0.8% a year on average in real terms during this Parliament in England - costs in the NHS are rising even more quickly.
What is more, many argue the cuts to social care, which includes everything from care homes to help in the home for tasks such as washing and dressing, has had a knock-on effect. As a result, there is a growing consensus that how the health and care sectors are arranged needs re-thinking so they are much more joined-up.
What are the numbers?
NHS funding in England rose from £100.4bn in 2010-11 to £113bn in 2014-15 - a rise of 0.8% a year in real terms, while social care spending was £16.1bn in 2008-9 and £17.2bn in 2013-14 - a fall of 3% a year in real terms.
Spend per head was £1,912 in 2012-13 in England; £1,701 in Wales; in Northern Ireland £1,850 and in Scotland £2,105.
The number of staff (in full-time equivalent terms) working for the NHS in England rose by 11,000 to 1.07 million between 2010 and 2014.
The most recent British Social Attitudes Survey shows 65% are "very" or "quite" satisfied with the NHS - the second highest level ever.
The NHS is expected to see 95% of A&E patients in four hours. That has not been met on a weekly basis since September.
What the politicians won't be saying
Two things: we want to reorganise the NHS, and, we can't afford to pay for care. Both are taboo subjects.
The government's reorganisation (see below) has given changing the way the NHS works a bad name. Even though the challenge of joining up health and social care is - in many ways - a reorganisation in all but name.
Politicians are also falling over themselves to promise more money for the NHS. But no-one will be saying they can't keep affording to provide the current range of services.
In the last year, many hospitals have been running up deficits. A number of radical solutions have been put forward - increasing taxes, introducing charges and rationing services more. But these are not topics the political parties want to discuss.
What has happened since 2010?
The last five years split neatly into two:
- first half was taken up by the government's reorganisation of the NHS in England, which the-then NHS chief executive Sir David Nicholson described as being so big it could be "seen from space"
- regional health authorities and more than 150 primary care trusts scrapped and replaced by new national body, NHS England, and a network of 211 clinical commissioning groups by the Health and Social Care Act 2012
- since then, debate has focused on how the health service has been performing - pressures have been building, causing key targets to be missed
- the 62-day target for cancer patients to be treated following an urgent referral has been missed for a year
- last three months of 2014 saw A&E waiting times in England drop to worst level since the four-hour target was introduced a decade before
- hospitals have been struggling to see patients who need routine operations, such as hip and knee replacements, quickly enough and last summer ministers announced a "managed breach" of the 18-week target to deal with a backlog of long waiters.
What do the experts say?
"All parts of the NHS are struggling with ever increasing patient demand, staff shortages and insufficient resources. Key areas, such as an A&E and GP services, are under particular stress and in some cases are close to breaking point." - Dr Mark Porter, chairman of the British Medical Association
"The position in the acute sector continues to be particularly difficult, with nearly three out of four hospitals currently in deficit. The prolonged slowdown in funding that the NHS is facing means that even well managed organisations are struggling to avoid significant debt." - Richard Murray, director of policy at the King's Fund think tank
"Pressures on the health and care system have never been greater. As the election approaches, we now need all candidates to play their part in a constructive debate about how to ensure health and care services are sustainable and deliver the best possible outcomes for people." - Rob Webster, chief executive of the NHS Confederation, which represents trusts