A spike in the number of deaths in England and Wales last year was due to an increase in dementia and respiratory diseases such as flu among older people, according to official figures.
The peak happened in the early months of 2015 when flu was common, according to the Office for National Statistics.
People with dementia were particularly vulnerable to flu, figures show.
2015 saw the largest rise in deaths in England and Wales in over a decade.
Last year there were 529,613 deaths registered in England and Wales, an increase of 5.6% compared with 2014, with 86% of the extra deaths occurring in the over 75s and 38% in the over 90s.
Claudia Wells of the ONS said: "The majority of the increase in deaths in 2015 happened during the first few months of the year, coinciding with an increase in hospital admissions for flu and reports of numerous outbreaks of the virus in care homes.
"Respiratory diseases, such as flu, were also mentioned in a third of deaths from dementia and Alzheimer's last year."
Prof John Newton of Public Health England added: "A range of factors can push up the number of deaths in older people in a particular year.
"An outbreak of flu can have a big impact, especially on those who are most vulnerable or experiencing other illnesses, such as dementia.
"An increase in deaths will generally lead to a decrease in life expectancy that year, but we have seen these annual fluctuations before and the overall trend has remained positive. "
Hilary Evans of Alzheimer's Research UK, said the report by the Office for National Statistics "further underlines the devastating impact of dementia and the scale of the challenge we face".
The main flu virus in 2015 was particularly dangerous to older people if they were infected.
There were outbreaks in care homes and admissions to hospital and intensive care for flu were higher than usual.
There was also evidence that the flu vaccine was less effective than expected.
The large increase in deaths in the early part of 2015 was not repeated in the early part of 2016.
In Scotland, registered deaths in winter 2014/15 were at their highest level since winter 1999/2000, according to National Records of Scotland.
In 2014/15 there were 870 excess winter deaths in Northern Ireland, the highest since 2009/10, the Northern Ireland Statistic and Research Agency found.
Jeremy Hughes, chief executive of Alzheimer's Society, said: "These findings serve as a stark reminder of the need for good community care to support the most vulnerable.
"People living with dementia often have a lowered immune system and so are at a greater risk of contracting flu viruses. The condition also makes it harder for people to look after themselves and in the cold winter months this can become a real danger."