The first study found that having flu ups your chances of suffering a stroke by nearly 40 per cent over the following 15 days.
That risk can last for up to a year after going down with the virus.
Led by Dr Amelia Boehme from the Vagelos College of Physicians and Surgeons at Columbia University in New York City, scientists looked at the patient records of nearly 31,000 patients who had suffered an ischemic stroke in 2014.
Study participants had an average age of 71.9 years old and were almost evenly split between men and women.
Scientists then compared each patient's "case window" – the time preceding stroke – to the time window for a set of control periods using the same dates from the previous two years.
Flu poses the same devasting risks regardless of age, gender or race
"We were expecting to see differences in the flu-stroke association between rural and urban areas," said Dr Boehme.
"Instead, we found the association between flu-like illness and stroke was similar between people living in rural and urban areas, as well as for men and women, and among racial groups."
While they haven't come to a definite conclusion yet, the researchers suspect that flu may trigger inflammation which then leaves people vulnerable to attacks.
A second study by the same institution found that people were at a higher risk from tearing neck arteries within one month of having a flu-like illness.
That's a leading cause of ischemic stroke in people aged between 15 and 45.
People are most at risk 30 days after having flu
This time, scientists reviewed 3,861 cases of first non-traumatic cervical artery dissection within the New York State Department of Health Statewide Planning and Research Cooperative System.
They found 1,849 instances of flu-like illnesses during the three years preceding neck artery tears.
Patients were more likely to suffer a flu-like illness within 30 days prior to tearing, compared to the same time one and two years before.
"Our results suggest that the risk of dissection fades over time after the flu," said the second study's lead author, Madeleine Hunter.
The studies, although based on a big group of people, did have its limitations.
"An important limitation of using an administrative dataset is that we had to rely on billing codes to determine who had cervical artery dissections, influenza and flu-like illnesses.
"If a diagnosis was not coded or miscoded, we could not capture it," Hunter said.
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