Take 19-year-old Helen. She had been doubly unwise during her travels in Africa. She had come to our Department of Infectious Diseases with bilharzia, a nasty bowel or bladder parasite that can cause cancer, contracted from cooling off in Lake Malawi. As if that weren't bad enough, as she was leaving she delivered a "while I'm here, doctor" (we love this) from the doorway. It was some itchiness and irregularity on her scalp, behind her right ear, which no amount of her chemist's creams had been able to shift.
Like most young people seeing a bit of the world in those long, penurious months before her course began, she'd been roughing it. She was now paying the price of deserting the crisp sheets of her childhood bedroom in Weybridge. There was no mistaking the scaly, bumpy tracks with their dried and crusty surface. Not an unusual infection - just at the wrong end of the body. The characteristic wandering skin bugs of cutaneous larva migrans usually infest the feet, hands or abdomen, which most often come into contact with infected soil. Helen's sleeping habits had caught her out when she rested her head on the worm-ridden red earth of Malawi, entranced by the brilliance of the African night sky.
With the ease of modern travel, the increasing tendency of universities to insist on a year to mature before matriculation, and the eagerness of both the young and not-so-young to backpack around the world, such parasitic infections are becoming more common. Some experts believe that they may increasingly begin to become endemic in Britain and northern Europe as the climate changes. Lyme disease, an unpleasant infection caught from deer ticks that can cause arthritis and even paralysis, is reportedly spreading through the UK. And we have already had the first rabid bat on the south coast of England.
Not all parasites are so innocuous as the readily cured itchy skin irritation of cutaneous larva migrans. It's often forgotten that malaria used to be endemic in Britain, and even killed Oliver Cromwell; occasionally, a vindictive and persistent mosquito will slip through on an incoming flight and infect people who live near airports or handle baggage. And even malaria can seem positively benign compared to some of the others. If you are of a squeamish disposition, it's probably best that you don't read on. Just like The Fat Boy in The Pickwick Papers, I wants to make your flesh creep.
Take Callitroga hominivorax, literally "maneater". An African fly lays the larva of this startlingly savage creature, straight out of the film Alien, inside your nose. It can chew its way through to your nasal bone within hours. The nose and mouth may be entirely devoured and the larvae then gnaw right down to the brain itself. You then, probably mercifully, die of meningitis.
And there are plenty more. Helen's simple skin hookworm has a far more vindictive cousin called Ancylostoma duodenale. Not content with just coursing through your skin, it migrates into your gut and lungs, and can kill. It is probably the leading cause of anaemia in the developing world. Horse-owners may be familiar with something similar. The eggs of the tropical warble fly, Dermatobia, are usually laid on the coats of their horses and cattle. Most grooms will be familiar with the fine-toothed combs used to scrape off the tiny, sticky yellow eggs in the summer months. When they infect humans, they may be removed with a scientific variation on the old tapeworm and Mars bar joke. A rasher of bacon is taped to the skin immediately over the worm's entry point. The maggot-like larva prefers the dead tissue and burrows into it. Best to discard the bacon afterwards!
Had enough yet? Elephantiasis, the gross distortion of limbs or genitals due to obstruction of the lymph drainage by the parasite filaria, is still comparatively common in Africa, India and South-East Asia. Swelling can be so bad that sufferers sometimes need to carry their scrotums about in a wheelbarrow. The larvae of loa-loa, the cause of painful, lumpy "Calabar" swellings, can sometimes be seen swimming through your eyeball. River blindness, a similar fly-borne parasite, is, sadly, one of the world's most frequent causes of visual impairment, though it is highly preventable.
The kissing bug sounds harmless enough, but in South America it defies its innocent name by defecating in your mouth as you sleep. The parasite that it spreads, a relative of sleeping sickness, poisons the nerves of your gut and heart so that you die either of heart failure or aspiration of your stomach contents. There are many, many more - I could go on indefinitely, but perhaps you are having your lunch.
Fortunately for Helen, her infection was harmless and responded to treatment with the relatively benign drug thiabendazole. You can readily pick up ancylostoma, a worm like Helen's, on Brighton beach if dog-walkers have not bothered with their pooper-scoopers. Compared to some of the other risks of foreign travel - HIV infection, accidents, malaria, assault and more common bacterial and viral illnesses - the chances of infection with such exotica are still fairly small. Probably still worth the risk to escape Weybridge, even temporarily.
The writer is a research fellow at the Centre for Infectious Diseases in central London
Reduce the Risk of Holiday Bugs
l The deadliest parasite you are likely to encounter is malaria; about 2,000 cases are imported into Britain each year, and it can kill. Get advice from your GP, a local travel clinic or the British Airways Travel Advice Centre
for prophylaxis at least a month before you travel, as well as for other local infection risks. Don't rely on taking tablets - much safer to avoid being bitten by using nets, repellents and full-length clothing.
Don't swim in tropical and sub-tropical lakes and rivers, particularly Lake Malawi. Lake Malawi is to bilharzia what Henley Regatta is to toffs. Drop in only if you want to mix with them.
Don't sleep directly on the ground - use a sleeping-mat.
Wear shoes or sandals.
Duck-hunters should take particular care. The ground-hugging stalking of prey is very effective at transmitting parasites under your skin.
Use a condom if you are going to have casual sex.
Beware uncooked vegetables and salad items, especially lettuce. Human waste is sometimes used as a fertiliser. Make sure meat is properly cooked and drinking water boiled, filtered or sterilised.
If you develop a fever or unusual symptoms on your return, contact your doctor as soon as possible.