A study by Swiss researchers found that not only do heart attacks increase when the weather gets cold, but also the lower the temperature, the higher that number climbs. The team also discovered that winter heart attacks are usually more severe.

One important factor is that cold weather makes blood vessels constrict, which plays a role in clot formation, and can lead to both heart attack and stroke. But that’s not the whole story.

At age 80, my patient Rachel had seemed healthy and always energetic, with a vibrant spirit that endeared her to friends and family alike. She also rarely got sick, until one winter she came down with a severe case of the flu. It pretty much knocked her out.

“I slept round-the-clock for two days,” she told me. “This flu took away all my energy. It’s the first time that’s ever happened,” she said.

Still, Rachel appeared to be on the rebound. But a few days later, I learned she had died in her sleep.

She wasn’t the only person I heard of who suffered an unexpected heart attack or cardiac arrest that winter. And one common denominator between them was a case of the flu.

Any type of upper respiratory virus ratchets up inflammation in the body, and that immune response is now considered a main driver in heart attacks.

In fact, a recent study from Australia found that heart attack risk goes up 17-fold in the first week following a respiratory infection. And although that risk goes down after the first week, it remains elevated for an entire month, according to the research.

For the study, 578 patients who had suffered a heart attack due to a blocked coronary artery were asked about their activities prior to the heart attack, including whether they had experienced a flu-like illness.

Symptoms included sore throat, cough, fever, sinus pain, as well as if they had received a diagnosis of pneumonia or bronchitis.

A second analysis was restricted to subjects who had symptoms in their upper respiratory tracts, including common cold, pharyngitis, rhinitis, and sinusitis.

For those patients who reported mild upper respiratory tract infection symptoms, the risk was less, but was still 13 times higher than for those who hadn’t gotten ill.

Of course, flu, upper respiratory infections, and colds are most prevalent during the winter months.