The vast majority of these nasties don't originate in Hong Kong, but, say medical experts, are picked up on the mainland, in Southeast Asia, or in developing countries. Hong Kong then becomes the 'get to know you' point for the parasites and the travellers who are carrying them.

'The most common means of infection is by ingestion,' says family GP Michael Cheng. 'The food or water is contaminated with the parasites or their eggs. They attach themselves to the intestinal wall, and sometimes migrate to other internal organs.'

'Based on our clinical experience,' says KH Kung from the Surveillance and Epidemiology Branch at the Centre for Health Protection in Kowloon, 'common parasites include roundworms, pinworms, hookworms, tapeworms and liver flukes.'

The parents of five-year-old Sam Jones rushed him to a hospital emergency room at 11pm, after three nights of continual crying and rectal pain. 'We just couldn't work out what was wrong,' says Sam's mother, Alice. 'He would be fine all day, but when it came time to sleep, he would start crying and complaining of pain in his bottom.

'I thought perhaps it was haemorrhoids or constipation. We tried giving him lots of liquid, and soaking him in a hot tub to relax the muscles, but nothing helped. I couldn't see anything obviously wrong.

'At the hospital they took a torch and showed us the tiny white worms - pinworms - in his anal passage. They're dormant by day but at night they wiggle, causing itchiness and sometimes pain.'

Itching around the anus, disturbed sleep and irritability are common symptoms of pinworms. If the infection is severe, symptoms may also include loss of appetite and restlessness. Most symptoms are mild and many infected people have none at all.

Sometimes, realising there's a problem is the hardest thing about finding and curing a parasitic infection. Most of us think we've come back from holiday with just a mild tummy bug.

Thirty-eight-year-old Maya Twardzicki began suffering from bloating, wind and bouts of diarrhoea following a holiday to Thailand in 1995. She continued to feel ill for two years, and her symptoms grew to include sporadic rashes and fainting. 'I was living in London at the time, and my local doctor had absolutely no idea what was causing the problem. After six months of failed treatments he concluded it was Irritable Bowel Syndrome (IBS).

'When the treatment for this didn't work, I went to the London School of Hygiene and Tropical Medicine. Almost immediately, they detected severe giardiasis infection, but by that stage the parasite had wreaked substantial damage. As a result of the infection and the heavy doses of antiobiotics I was left with a leaky intestine - which means I have problems digesting foods - lactose intolerance and an overgrowth of candida. However, after going on an anti-candida diet and taking recommended supplements, things are getting better.'

Giardiasis is a parasite that lives in the digestive tract, and is a common cause of gastroenteritis, which infects about 200 million people worldwide. Problems occur when the parasite coats the inside of the small intestine and blocks its ability to absorb nutrients. Symptoms include loss of appetite, lethargy, fever, explosive diarrhoea, stomach cramps, bloating and wind. Typically, symptoms appear one or two weeks after infection, and may wane and reappear cyclically. Giardiasis is spread via the fecal-oral route, usually from personal contact and contaminated water and food. As the parasite only tests positive at certain stages of its life cycle, it's important to have periodic tests to ensure detection.

Amazingly, only about a third of people infected with giardiasis exhibit symptoms, and treatment is not always necessary: in many cases the body can defeat the infection by itself. Cheng warns, however, that some parasitic infections can cause problems if left untreated.

'Parasitic infections are rarely fatal, but they may cause anaemia and malnutrition. I advise travellers to see their doctor if they develop fever, persistent diarrhoea or abdominal discomfort during or after their trip. Treatment usually involves medication such as antibiotics for amoeba, giardia or campylobacter, and there are specific treatments for worms such as schistosomiasis.'

Downes and Jones got rid of their parasites by taking tablets. Both were cured quickly and easily, although Downes' foot took nearly four months to return to normal.

'As soon as I started taking the tablets,' she said, 'the red lines disappeared, but my foot looked withered and wrinkly and my skin remained very dry.'

In the vast majority of cases the cure is as simple as taking a pill, but it's important to watch for potential complications. Following a holiday in Japan, six-year-old Max Kennan developed persistent vomiting, diarrhoea, headaches and weight loss. He was hospitalised twice for re-hydration and intravenous antibiotics, and diagnosed with a fairly common bacteria, campylobacter, which causes travellers' diarrhoea. Unfortunately, three weeks later he developed severe complications in the form of Reiter's syndrome, a rare condition.

'His body reacted to the bacteria by going into immune overdrive, and started attacking healthy cells thinking they were the bacteria,' says his mother, Anna. 'He had an alarming rash, a high fever that spiked and fell throughout the day and night, uveitis (inflammation of the iris) - and his blood tests revealed liver inflammation. His body went haywire in response to the bacteria that had been in his system weeks before.

'It took a long time to diagnose as he was being treated with antibiotics for a bacterial infection, and the doctors couldn't work out why he wasn't improving. He's recovering now and we're hoping this has been a reactive auto-immune response rather than a longer term problem.'

So how can you avoid contracting these unpleasant freeloaders? 'Since the most common source is contaminated food or water' says Cheng, 'simple food and hand hygiene is the best way for prevention. Make sure the food is washed properly in clean water, and all food is cooked thoroughly. Wash your hands before touching or preparing food, and don't risk mixing raw and cooked foods during preparation.

'All this is easy at home: the hard part is when you're eating out, or travelling. I've seen amoebic dysentery in travellers who have eaten only at five-star hotels.'

Downes has her own advice for travellers: 'My doctor said that schistosomiasis breeds in the warm sand, so my tip is to always wear flip-flops on the beach.'