It started suddenly: so suddenly, in fact, that Gary - my husband - thought he had food poisoning. Feeling lousy, shivery, looking pale, he went to bed. Clearly he wasn't going to work tomorrow, but equally clearly he would be fine in a day or two.

Except that he wasn't. Two days became four, and then six. Sometimes Gary seemed a bit better: one night he watched a video and said he felt OK, that he was definitely on the mend. And then the next morning he would emerge from the bedroom again saying he had had another dreadful night - I had moved out of our room to sleep with our two youngest children who themselves had bad colds. As far as I was concerned, Gary simply had a worse version of what they had, and I was becoming mildly irritated that he wasn't snapping out of it while I was holding the fort with our four kids.

On day five Gary made valiant efforts to get me to take him to the doctor. I did call the surgery, but the receptionist was of the same frame of mind as me: when I described his symptoms (feeling awful, bad chest, sore throat etc) she agreed it was definitely flu, and confirmed there was no point in wasting the doctor's time.

Crunch time came two days later: Gary came downstairs saying he had just had the worst night of his life, and looking across the room I realised he was the same shade of puce as the wall behind him. It was time to take him to the doctor, although I still thought he would be sent home with antibiotics. When I went round to pick him up after his appointment he was standing on the pavement looking about 60. "Hospital," he muttered weakly when he got into the car. "I've got to go straight to hospital."

The outpatients' department at St George's in Tooting was packed with seriously ill-looking people, so it was a bit scary to find our GP had phoned ahead and that Gary had been prioritised as an urgent case. He seemed a little better once he was on a trolley going off for a chest x-ray. But that's not what the nurse thought when the pictures came back. "How on earth did things get this bad?" he said, as the doctor diagnosed it as a definite case of pneumonia.

How on earth did Gary - an otherwise fit 41-year-old - get so ill? What were the symptoms we had missed that should have told us this was more than a simple case of flu? According to John Oxford, professor of virology at Queen Mary's School of Medicine in London, the crucial warning sign that Gary and I overlooked was that he felt he couldn't get enough air into his lungs. "You can put up with a sore throat," he says, "But any sort of shortage of breath when you go up and down stairs, or breathing difficulties or dizziness, and that's a sign to get help immediately."

Pneumonia is caused by a microbe - it can be viral, though more usually it is bacterial - getting into the lungs and causing a fluid build-up. "The doctor will pick up on a shortage of breath, a raised temperature, that you're feeling ragged," says Oxford. "When a diagnosis is made, in nine out of 10 cases antibiotics will sort it out."

In Gary's case, intravenous antibiotics made a huge difference within a day or two - although that didn't stop him spending most of a week in hospital, and needing a month off work. It was all a huge shock: why had he succumbed to such a serious illness at all? Apparently there were different possible scenarios: Oxford says infections affecting the respiratory system are so complex that they are difficult even for doctors to unravel. It's possible that Gary's original illness was flu - a virus - that weakened his immune system and made him vulnerable to pneumonia-causing bacteria. These opportunistic little monsters appar ently hang around the respiratory tract of healthy people waiting for their defences to go down. Or he might have started out with bacterial pneumonia from day one.

Most people who get pneumonia are vulnerable to chest infections of all sorts - they are, broadly speaking, people over 65 and babies under two, like the four young children who were reported to have died of flu last week in England and Scotland. But, says Oxford, every year a small number of entirely healthy individuals outside the risk groups, such as Gary, are unlucky enough to go down with pneumonia against the odds. Some even die of it - I heard several tragic stories from friends after Gary's brush with serious illness.

Mark Britton, a consultant chest physican who chairs the British Lung Foundation, agrees it is notoriously difficult to know when a cold is a cold, or flu, or even bronchitis, pleurisy or pneumonia. "The crux of the issue is that if flu is going around and you get an illness that fits the bill then it's most likely to be that," he says. "But if it lasts longer than three days or so, and symptoms don't settle, then the chances are that you've either not got flu or it's gone on to be something more serious, and you should see a doctor."

In fact, this winter's victims of flu are luckier than last: for the first time a new anti-viral drug, Tamiflu, will be properly available. And although it's people in the vulnerable groups who are highest priority for these drugs, it's likely, says Oxford, that anyone like Gary who turned up with a bad dose of flu that looked like it wasn't getting better on day three or four would probably get a prescription. What that means is that, even if our doctor's receptionist was right last year to say there was no point in bothering the doctor with flu on day four, it certainly would be worthwhile this year. (In fact, the moral of the tale is to never, ever, base your decision about whether to consult a doctor on the receptionist's hunch.)

The big question of this winter is whether the dreaded Sars pneumonia will return. (There was actually only one case in Britain last year, but more elsewhere and a lot of fear.) Oxford believes that particular virus is now endemic in south-east Asia, and that it may be biding its time to make a comeback: his only word of consolation is that Sars is more the dachshund of pneumonia outbreaks than the greyhound, by which he means we are unlikely to see an epidemic racing out of control. In my own family, meanwhile, symptoms of winter illness will never be quite so easily dismissed again. We hope not to be forever in the GP's waiting room, but we certainly don't want to be back in A&E.

So what's your chest infection?

Common cold

One of around 100 different viruses causing stuffiness and irritation in the nose.

Symptoms: Sneezing, runny nose, scratchy throat, coughing. It lasts from two days to two weeks.

What to do: Carry on as usual - take paracetamol to relieve symptoms if bad or try a hot toddy at night. Stay off work if you feel really unwell, and only go to the doctor if you are running a high, sustained temperature or coughing up sputum.

Will it kill me? No. May escalate into acute bacterial sinusitis, which needs treatment (suspect that if you are no better after eight to 10 days) and it can weaken your immune system, making you more vulnerable to other infections.

Influenza

One of a large number of viral infections affecting the respiratory tract.

Symptoms: You're so ill you can hardly get out of bed. You've got aches and pains all over - not to mention a headache, cough, raging temperature, chills, sweats and a sore throat.

What to do: Drink lots of water and stay in bed. Keep warm and take paracetamol. If you are in a vulnerable group (the elderly, very young or with an underlying chest condition), or if it continues for three days or so without abating, see your GP as s/he may prescribe the new anti-flu drug.

Will it kill me? 3,000 to 4,000 people die annually of flu complications such as pneumonia.

Pneumonia

A bacterial or viral infection causing fluid build-up in the lungs.

Symptoms: It starts suddenly with shivering fits; you have pains in the chest and a cough, dry at first but later phlegmy. It starts to hurt when you breath in deeply.

What to do: Go to your GP as you need antibiotics.

Will it kill me? It can be fatal, especially to vulnerable groups.

Bronchitis

A bacterial infection in the major airways into the lungs.

Symptoms: You're coughing up lots of foul-smelling sputum and you're a bit feverish and breathless.

What to do: See your doctor: you need antibiotics

Will it kill me? Continued bouts, which may happen in people who are vulnerable because of chest conditions, can lead to lung damage which may, in turn, make you more likely to contract pneumonia.

Pleurisy

Swelling and irritation of the lung lining.

Symptoms: Sudden, severe, stabbing chest pains, especially when you take a deep breath, and usually on one side. Also fever and maybe shallow breathing.

What to do: Go to your GP, as it tends to get worse and you might have pneumonia.

Will it kill me? In that it can lead to pneumonia, yes.