Constantly struggling to breathe and feeling as though he was "drowning" because he "simply couldn't get enough air in my lungs", adventurer and TV presenter Ben Fogle feared his apparently failing health would bring his physically challenging career to an end.

Although the 43-year-old - known for having hosted Countryfile and New Lives In The Wild on television - is no stranger to health hazards and has battled everything from dehydration, heat stroke and frost bite, to a flesh-eating bug while pushing his body to extremes for 'work'.

However, he reveals he was totally unprepared for the debilitating effects of a bout of pneumonia three years ago.

‘My body truly let me down’

"It's the first time in my life that my body's truly let me down. At one point, I wondered whether I'd ever be able to get a full breath again. I couldn't walk up a flight of stairs without feeling breathless. For someone whose life is totally active with anything from abseiling to diving or climbing mountains just part of a day's work, I was suddenly reduced to only really feeling comfortable when I was sitting down," recalls Fogle.

"It was very scary because being physically fit is essential for my career, and pushing myself to the limits is at the heart of what I do. Suddenly I had to come to terms with the fact my health might be permanently compromised, and I might not be able to work like that again. It really rocked my confidence."

Initially, Fogle - who's supporting pneumonia awareness campaign, Expect The Unexpected, funded by Pfizer - was dismissive of a headache, cold, and cough he developed in 2014 during a break from filming ITV's Harbour Lives, to holiday in Portugal with his wife, Marina and children, Ludo, seven and Iona, five. A week in and he was forced to go to hospital where he was diagnosed with pneumonia and immediately treated with antibiotics.

Shocked by the diagnosis

"The frightening thing about this illness is that it sort of sneaks up on you and knocks you for six. From just feeling rather groggy, I suddenly felt dreadful. Over a few days, my breathing became very laboured and I woke up one morning with my chest hurting so badly it was as though I'd broken a rib," he explains.

"Even then I assumed I just had a bad cold which had turned into a chest infection, so I was totally shocked by the diagnosis. I honestly didn't think people of my age, or as healthy and fit as me, could suffer from an illness that came out of the blue."

Pneumonia is when the tissue in one or both lungs becomes inflamed, often having been triggered by a bacterial infection. The tiny air sacs at the end of the lungs also become inflamed and fill with fluid.

Symptoms include a dry cough, accompanied by thick mucus, difficulty breathing or breathlessness, fever, increased heart rate and a general feeling of being unwell.

Doctors explained to Fogle that he had "fluid filling one lung, which is why I constantly felt as though I was drowning and struggling to breathe. It was like trying to cope at high altitude and being oxygen deprived. They told me treatment was essential otherwise the other lung could fill with fluid and then my health would be totally compromised and I'd be on the edge," he says of the infection, which necessitated three courses of antibiotics to bring it under control and even then, three months later, he suffered a further chest infection, which delayed his recovery.

"Ironically, I'd felt incredibly fit prior to it happening. I was working really hard - and had been flying backwards and forwards filming in Africa as well - but I'm very health-conscious and try to exercise for at least an hour every day," he explains.

[Read more: Pneumonia: What is it and how to treat it]

Relying on his fitness

Fogle, who's relied on his fitness levels and resilience to get him through punishing personal challenges in the past, including racing to the South Pole, rowing the Atlantic, and taking part in the Marathon Des Sables, says although his breathing recovered after around eight weeks, the effect on his life and career were longer lasting.

"It was incredibly frustrating having to cut out anything which I wasn't physically capable of. Diving, running a half marathon and doing a training course with gamekeepers protecting elephants in the African bush were just a few of the things that had to be dropped," he says.

"Despite all those precautions, for a long time afterwards, every time I tried to do physical exercise, like going for a run, a bike ride or a session in the gym, I could feel a rasping and slight wheezing in my lungs. I didn't feel I could afford to take any time off because, as a freelance, I have to take the work when it's around and I have a young family to support, but, with hindsight, I don't know how sensible that was. I probably slowed down the healing process by not resting."

[Read more: Thinking about private health cover? Get a free, no obligation quote]

Worse than a flesh-eating bug

After nine months he felt fully recovered, which he describes as "a massive relief because I was finally able to put the ordeal behind me".

Despite the fact he's suffered with the flesh-eating bug, leishmaniasis, which he contracted while filming a BBC series in the Peruvian jungle in 2008, and had to undergo two courses of chemotherapy, he regards suffering pneumonia as "worse".

"They were both pretty awful, but the effects of the pneumonia took far longer to shake off," he says. "I never want to get either of them again. Nowadays, I'm actually probably pushing myself even harder and working more than I was when I got the pneumonia. But I'm ultra-conscious of my health and the need to look after it and want to warn other people never to ignore symptoms or underestimate this illness."

Ben Fogle is supporting the Expect The Unexpected pneumonia awareness campaign funded by Pfizer. For more information visit