More people died after being admitted to Calderdale and Huddersfield NHS Trust with pneumonia than any other condition, figures reveal.
New NHS data shows 395 people died after getting the infection in 2018, in which lung tissue becomes inflamed and fills with fluid.
It means that of the 2,460 deaths over the year in the trust's hospitals, or up to 30 days after patients were discharged, 16 per cent were linked to a diagnosis of pneumonia.
The British Lung Foundation said that between five and 11 adults in every 100,000 get pneumonia each year in the UK, with over-65s particularly vulnerable.
A BLF spokesman said: “There are things people can do to reduce their risk of pneumonia, such as not smoking, and there are vaccinations available to protect those who are at higher risk."
The picture at Calderdale and Huddersfield NHS Trust is reflected across England, where more people also died after being diagnosed with pneumonia than any other condition.
Across England, around 15 per cent of the 293,000 patients who died last year were initially diagnosed with the condition.
Nick Scriven, president of the Society for Acute Medicine, said the data confirmed "the massive stresses the health care system is under", and that it reflected an older and frailer hospital population.
Commenting on the pneumonia figures, he added: "The stress to the system is added to by the fact that this sort of illness is not one that is treated in 24 to 48 hours in the elderly, and often they will need longer times in hospital and then time recovering."
The second most common initial diagnosis for deaths at Calderdale and Huddersfield NHS Trust was septicaemia, or blood poisoning, with 205 patients dying.
Acute cerebrovascular disease - a cause of stroke - was third, with 145 deaths.
Urinary tract infections, which affect the bladder, urethra or kidneys, and acute or unspecified kidney failure were also common diagnoses.
NHS Digital, which released the data, said the reason a patient was admitted may not directly correspond to their cause of death.
Across England, the top five diagnoses resulting in deaths in or after hospital were:
Pneumonia (15 per cent)
Septicaemia (11 per cent)
Acute cerebrovascular disease (6 per cent)
Heart failure (4 per cent)
Aspiration pneumonitis (3 per cent)
The death rate at the Calderdale and Huddersfield NHS Trust was categorised as “expected” in 2018, with the difference between the number of deaths and the total expected based on averages across England falling within the anticipated range.
However, the NHS says the rate should be looked at alongside other measures when judging the performance of a trust.