‘Stress has a well-established effect on your immune health,” says Daniel M Davis, head of life sciences at Imperial College London. “But stress is a very broad phenomenon. You can feel stressed watching a horror movie, or you can experience long-term stress, like going through a divorce.”

Short-term stress can temporarily affect your immune system. “The number of immune cells in the blood changes,” says Davis. “But it returns to normal within about an hour, so it’s unlikely to have any major impact.”

Long-term stress is a different story. When your body perceives a threat, the adrenal glands release hormones such as adrenaline and cortisol. They prepare the body for a “fight or flight” response – increasing your blood pressure and heart rate, but also suppressing the immune system.

“We can see this even at the molecular level,” says Davis, the author of Self Defence: A Myth-Busting Guide to Immune Health. “If we take immune cells and expose them to diseased cells, like cancer, the immune cells will usually destroy them. But if we add cortisol, they become much less effective.” If this exposure to stress hormones lasts for weeks or months, our immune system will be diminished.

Practices such as mindfulness, tai chi and other stress-reduction strategies have been shown to lower cortisol levels, which should benefit immunity. However, proving this is difficult, says Davis. You can’t ethically expose large numbers of people to an illness to test their immune response, and measuring “immune health” in a real-world setting is complicated. “It’s also hard because you can’t avoid stress,” he adds. “Every change in our life will have some stress associated with it, and a lot of changes are positive.”

That said, if you’re worried about long-term stress, Davis recommends reaching out to your GP.