Last week I saw a person with chickenpox. That doesn't sound like news - but he was 35! Everyone assumes that chickenpox is a child's disease, and that by the time we are adults we have had it, and are immune to it. Not so. My patient had the full blown illness, with spots all over him - and he was quite ill.

Chickenpox is the most infectious of all the childhood diseases, and there's no vaccine against it yet. He had grown up on a farm, so he must just have been one of the very few never to have come into contact with the virus, purely by accident.

Getting a child's disease as an adult can pose severe problems. The worst is rubella, or German measles, which harms unborn children if their mothers catch it in the first two to three months of pregnancy. That's why it is so important to keep up our excellent vaccination rates in children. If acceptance rates for measles, mumps and rubella vaccinations dip, a lot more adults will eventually catch them. Measles and mumps can be devastating in adults - in particular they can cause a brain inflammation (encephalitis) that can produce permanent brain damage and even death.

Worse than getting a child's disease as an adult is getting an adult's disease as a child. We used to think of chronic bronchitis as affecting only people over 50 - but we're seeing more and more cases in people, particularly women, in their twenties. That's partly because so many more schoolgirls smoke, but it's also because there was a lot more whooping cough around 20 years ago. That was the result of a vaccine scare campaign.

The cardinal sign of chronic bronchitis (its modern name is chronic obstructive pulmonary disease, or COLD) is a smokers' cough. You almost don't notice it for many years, but it's always with you. And while you have it, bit by bit, the smoke is destroying the most delicate part of the lung - the tiny air sacs through which oxygen passes into the bloodstream. So DON'T ignore that first-thing-in-the-morning cough. If you don't stop smoking once you get it, there's more than a fifty-fifty chance it will kill you eventually.

Only five in every hundred adults with COLD are non-smokers. Three have had severe chest infections as infants, the other two have a genetic problem that damages the lung. It's called alpha-1-antitrysin deficiency (A1AD), and it's one reason for the intense interest in Dolly the sheep. Her offspring could well soon be producing in their milk the missing chemical that sufferers from A1AD need.

Then there are diseases that used to be thought exclusive to women, like migraine. Now we are seeing it almost as often in men. It seems that men did have migraines, but didn't complain of them. Maybe they were a bit self-conscious of having a "woman's complaint?" The doctors at London's City Migraine clinic have now shown that city gents were just as prone to it as their female colleagues, and need the same treatment - a rest for a few hours in a dark room and effervescent painkillers to settle the stomach and head. The idea that migraine is caused by stress - an illness of the manager but not of the shop floor worker - is gone, too. Everyone can get it, stressed or not.

Then there's osteoporosis. It's supposed to be a woman's illness, but older men get it too. A lot of older men who live alone give up on cooking properly for themselves - and they can lose calcium from their bones just as easily as women. So if you care for a neighbour or older relative, watch out for the signs - a stoop, aches and pains, difficulty in walking, loss of weight, a distinct slowing down in the ability to move around. Osteoporosis is always treatable nowadays, so don't just think it's old age.

Cancer can hit people unexpectedly, too. A very small number of men get breast cancer, so a lump behind the nipple in men should be seen just as urgently as one in a woman.

As I write I have in front of me a letter from a remarkable woman who had bowel cancer when she was 28. She's now 32 and clear of the disease, but its original diagnosis was delayed because her doctors couldn't accept that her symptoms, at such a young age, could be due to cancer.

It's a hard lesson for doctors, but we must learn it. If the symptoms and signs of an illness don't quite fit the expected, we must think about the unexpected - and sometimes the unthinkable.

Converted for the new archive on 30 June 2000. Some images and formatting may have been lost in the conversion.