Vitamin pills and icy swims: Can you health hack your immune system?

It is a cold morning in the depths of winter.

And I feel like I’ve left my sanity as well as my warm clothes in the changing room as I stride out to the edge of a reservoir in my swim shorts.

A brightly-coloured chalk sign informs me that the water temperature today is a chilly 3.9C, as one of the regulars tells me this is not cold water swimming, it’s “ice” water swimming.

How did I end up here?

Well, I’ve become enthralled by the idea of enhancing or boosting the immune system. My body has been the living embodiment of the “quademic” that the NHS spent all winter warning us about. It’s been a relentless stream of colds and coughs and one explosive tummy.

Our immune systems already do a fantastic job fighting viruses and other nasties. If I collected all the air I breathed out over the course of a minute it would contain 100 to 10,000 bacteria, 25,000 viruses and a single fungi, according to Prof John Tregoning, immunologist at Imperial College London.

“You’re breathing these things in all the time, there’s just a swirling mass of pathogens [organisms that cause disease] in the air,” he says.

But there’s plenty of foods, supplements and activities that are touted for their “immune-boosting” properties. Can we dial up our protection?