Common respiratory and urinary tract infections play a role in triggering heart attacks and strokes, a study of 40,000 medical records suggests.
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Infections such as bronchitis, pneumonia and cystitis raised the risk of a heart attack fivefold, and of a stroke threefold, for the period a patient was ill.
The blame has been pinned on inflammation caused by the infections.
The research, funded by several leading research bodies, is published in the New England Journal of Medicine.
Scientists discovered the risk of heart attack and stroke was substantially higher in the three days after a respiratory tract infection.
The likelihood of having either then decreased gradually over the following weeks.
Researcher Professor Patrick Vallance, of University College London, said the work showed the timing of a heart attack was not always random.
He told the BBC News website: "After the age of 50, we all have some degree of furring up of the arteries, but most of the time it sits there fairly harmlessly.
"However, during infection stable deposits become unstable and may break off, causing the blockages that may lead to a heart attack or a stroke."
Professor Vallance said the idea that a patient's risk was constant was misleading. In fact, the risk fluctuated all the time.
He said the latest study suggested that extra care should be given to patients who were likely to be experiencing an inflammatory response - for instance those who were undergoing abdominal surgery.
Dr Liam Smeeth, lead researcher on the project, said: "This knowledge will open up new avenues for research and discovery.
"Armed with the information we have found, we can begin to develop new strategies to reduce the occurrence of heart attacks and strokes."
The team also investigated whether vaccinations, including flu and tetanus, increased the risk of heart attacks or strokes. They found no heightened risk.
Dr Douglas Fleming, from the Royal College of GPs, said: "It has always been the case that December is the key month for increased cases of bronchitis - particularly in children and the elderly.
"There has been a lot of argument on whether infections may precipitate heart attacks and it is true that, looking at the figures, the risk is slightly increased - but I would stress this is only a slight increase and patients should not be unduly worried."
He added: "As a rule, patients with slight bronchitis do not need to see their GP, however if their bronchitis is a problem then they should make an appointment.
"If a patient has a history of heart disease and the weather is particularly cold then they should try to stay indoors."
The study was funded by the British Heart Foundation, the Medical Research Council and the Wellcome Trust.