Those lunchtime swims were what kept me on an even keel in the bustle of my working day in central London.

While everyone else queued at Pret, I’d duck into Marshall Street Leisure Centre, a secret tucked-away bit of Soho where the city noise fell away.

For 40 quiet minutes under the barrel-vaulted ceiling, with daylight slanting across the tiles, I swam length after length until the stress loosened and the day reset. I always climbed out calmer, sharper, ready to work.

My undoing was in my rush to get dressed and back to my desk. Drying between toes feels like a luxury when you’re racing the clock, so I rarely lingered over the awkward bit.

It never occurred to me that the minutes I tried to save – hurrying past the showers, pulling on socks over damp feet – would be the ones that cost me weeks.

The trouble didn’t show itself straight away. It crept in later as a familiar, forgettable itch – the kind you file alongside a loose button or a squeaky hinge, something to ‘sort properly’ next week.

I carried a small pharmacy of half-used tubes in my rucksack: Canesten one month, Lamisil the next. Sometimes I’d shake Scholl antifungal powder into my shoes for good measure.

I knew it at once, because I’d had athlete’s foot before – a common fungal infection that thrives in warm, damp skin - especially if you don’t dry properly between your toes.

The itch would settle and I’d stop. Months later it would be back, and I’d start the ritual again.

But soon, the familiar nuisance changed character.

I was doing an MA at the National Film and Television School in Beaconsfield while freelancing as a journalist, when halfway through an afternoon seminar, I caught myself coiling and uncoiling my toes in my shoes.

A strange new sensation had arrived at the front of my right foot. It was not a sharp pain but a numb burn that made every step feel wrong, as if the floor had developed a tilt.

On the train back to London I kept curling and uncurling the toes, willing the feeling to go away. I had lived my whole life on the principle that if you ignore minor ailments they usually get bored and leave. This one did not.

By the next day I had acquired a comic-looking limp. My tutor, a former BBC natural history producer used to roughing it in the jungles of the Congo, watched me hobble in and gave me the kind of look usually reserved for someone asking for oat milk at a greasy spoon.

The hobbling got worse. After a couple of days, I went to A&E and sat waiting for hours while the heat in my foot moved in slow tides. I was convinced I had somehow kicked out during the night and broken a toe.

The X-rays were clear: there was nothing wrong with me. I was told to elevate the leg and rest, which I dutifully did.

A week later I was back at A&E, limping more heavily. It was no longer just an ache: my foot was swollen, hot and red; the edge of the rash creeping higher each day from the ankle.

A doctor finally conceded something more complex might be going on, though didn’t give it a name or any real treatment.

At home I tried to manage. Some days I forced trainers on, others I avoided shoes altogether.

Walking slowed to a crawl. I picked routes with fewer steps.

At night the heat kept me half awake. I didn’t feel seriously ill, just ‘off’. Showers stung. Towelling needed care.

On my third visit to A&E, nearly two weeks after I first noticed the pain, the diagnosis finally came: I had a bacterial infection in the skin and the tissue underneath, called cellulitis.

This explained the heat, the spread and why such ordinary things had become so difficult. The doctor gave me a course of antibiotics and told me to rest.

Almost as an afterthought they added that if it didn’t improve, hospital admission might be needed.

That word – admission – left me deeply alarmed, even as I tried to convince myself it was routine.

That evening my sister, who lives in the US, happened to call. I told her about my cellulitis, a term I’d previously never heard of.

She told me a colleague of hers had had it in their hip the year before and spent a month in hospital. Alarmed, I finally woke up to how serious my condition might become.

Reading about it that night, my blood ran cold.

Cellulitis, I discovered, is a bacterial infection that burrows beneath the skin.

The culprits are usually Streptococcus or Staphylococcus aureus – bacteria that often live harmlessly on us but can enter our bodies via cuts or bites.

A crack in the skin, from something as benign as athlete’s foot, is enough to let them in.

Changing rooms are the perfect vector: first for the fungus, then the bacteria. Once inside, the infection makes skin hot, red, swollen and painfully tender – and if left untreated, it can spread through the body.

Reading online, page after page spoke of patients on drips, nurses drawing lines on skin to track the spread, the word ‘sepsis’ appearing too often for comfort.

There were accounts of infections reaching a joint or a bone and taking months to clear.

After a few days of antibiotics, the improvement was dramatic and my natural optimism reasserted itself.

By the end of the seven-day course, the leg still did not feel completely right, but I told myself my body would naturally finish the job of healing.

A week later I was worse than before: the redness had spread again, the swelling was back, and every step sent a jolt of pain up my calf.

Determined to take it seriously this time, I immediately went to see my GP and told her the whole story. The GP glanced at the screen, at the clock, back at my foot.

When I asked for a second course of antibiotics, there was a small curl at the corner of the mouth, then the word I wasn’t meant to hear slipped out: ‘neurotic’.

Suddenly I wasn’t a patient with a documented infection; I was a fusser, a self-diagnoser.

After a pause she handed over the prescription. I walked out on my tender foot feeling humiliated and furious, as if I’d failed a test of character.

The second course of antibiotics worked slowly but steadily.

The heat eased first, then the angry edge softened, then the limp began to lift. The whole process took about six weeks. Six weeks of putting off a trip to the bathroom because you know the steps will sting, of worrying that the infection will return.

Doctors warn that although most cases of cellulitis are mild, worry is not misplaced.

‘Cellulitis can spread quickly and, if untreated, can become very serious – in the worst cases leading to sepsis,’ says Dr Sarah Morrow, a dermatology specialist registrar at Oxford University Hospitals.

‘Most cases are mild to moderate and usually clear with antibiotics if caught early. The important thing is to look after your skin and deal with the causes.’

Athlete’s foot can be a trigger, dermatologists say, but it’s not the only one.

‘The problem starts when cracks in the skin let bacteria in,’ explains Dr Leila Asfour, a consultant dermatologist and spokesman for the British Association of Dermatologists, ‘and that can lead to cellulitis requiring prompt antibiotics’.

A blister, an insect bite or a patch of eczema can open the same door.

My mistake was not taking the time to do dull things properly: not drying between toes, pulling on damp shoes because I was in a hurry – and stopping treatment when the itch faded. And not listening to my body sooner.

I’ve had flare-ups of athlete’s foot since but never anything like cellulitis again.

Now I do the dull things properly: I finish the antifungal course, rotate shoes, wear flip-flops in communal showers and act quickly if redness spreads or the skin turns hot and sore.

The minute I thought I was saving in the changing room turned out to be the minute that cost me six weeks.