Up to 20 million Britons are suffering from two or more serious health problems and the NHS needs restructuring to deal with them properly, according to a leading professor.
The number of people living with multiple long-term illnesses and dying prematurely is increasing, according to a report by the Academy of Medical Sciences, which suggests ineffective treatment is leading to early deaths.
"We are facing a tidal wave of patients living with multiple long-term health conditions, and our report demonstrates how little we know about how to manage this," said Professor Stephen MacMahon, chairman of the Academy of Medical Sciences multimorbidity working group.
"Outcomes appear to be worse in these patients and yet there is growing evidence that people with multimorbidity (several diseases at the same time) are less likely to receive appropriate care for the individual diseases they have.
"We face a situation where those in greatest need are least likely to receive appropriate care.
"For too long we've focused almost exclusively on the management of single diseases, such as cancer and HIV.
"This means we have neglected the reality that most people with any one long-term disease typically have others."
The report was worldwide, but the situation in the UK suggests multimorbidity is "the norm", according to Professor MacMahon.
An example given is the frequent co-existence of coronary heart disease and cerebrovascular disease - two similar diseases often found in the same person.
The report also shows physical and mental health conditions often cluster together and that poor mental health can negatively affect quality of life and life expectancy more so than having multiple physical illnesses.
The division between health services treating mental and physical health often means patients suffering from both physical and mental conditions are at particular risk of poor care, according to the report.
Dr Lynne Corner, director of engagement at the Newcastle University Institute for Ageing and Faculty of Medical Sciences, said: "It can't be a full time job, being a patient.
"We can't have five appointments in five days with five different teams in five different clinics."
She cited the example of a pioneering CRESTA (Clinics for Research and Service in Themed Assessments) clinic in Newcastle where one patient is seen by seen by multiple specialists in the same visit to the clinic.
Dr Corner said: "We are looking at models like that and seeing if we can roll them out more."
Although there are no precise figures for how many people in the UK suffer from multimorbidity, Professor MacMahon said a "ball park figure" is "roughly a quarter to a third" of the population, meaning 15 to 20 million people.
Melanie Davies, professor of diabetes medicine at the University of Leicester, said previous research has found that among over 50s the prevalence of multimorbidity steadily increased from 31.7% in 2002/2003 to 43.1% in 2012/2013.
Professor MacMahon argued more research is needed into how the NHS can restructure to better deal with patients with several severe illnesses, adding: "It really is urgent. It is not straightforward for the NHS to say 'this is how we should restructure.'
"Almost certainly restructure is required, but we just don't quite know at the moment how to do that."
A spokesperson for NHS England said: "Today's report is further evidence that England's NHS is right to continue driving integration in care, which is already delivering results across the country, including preventing strokes through better monitoring of high-risk patients by GPs and pharmacists, and integrated mental and physical health care for people with diabetes and other long-term physical conditions."
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