One of the simplest ways to avoid infection, therefore, is to avoid responding to the call of nature while swimming - although, as John Yu, specialist in dermatology at Matilda, says, the worst perpetrators are the hardest to control. "Always use a different pool for small kids because they tend to pee everywhere," he advises.

According to a recent study by China Agricultural University and Purdue University in the US, published in the journal , the implications go beyond the obvious unpleasantness of swimming around in pee. The researchers found that the uric acid in urine reacts with the chlorine used to disinfect swimming pool water to create cyanogen chloride and trichloramine, poisonous by-products that can cause everything from respiratory tract damage to childhood asthma, and can even cause long-term harm to the central nervous system and heart.

Fortunately, that very same chlorine, also used in the footbath you step through before entering the pool, kills a large number of the nasties that are swimming around with you - but not all of them.

"Most bacteria come from the gut, and most are killed by chlorine," says Dr Seto Wing-hong, consultant microbiologist in the pathology department of the Baptist Hospital.

"But there are some, especially parasites, that are chlorine-resistant - in particular cryptosporidium," one of the more common causes of gastroenteritis from dirty water, along with giardia, shigella, the norovirus and some of the nastier strains of .

Fortunately, there has never been an outbreak of cryptosporidium here, and like most of these pathogens, it's easily avoided - just make sure you don't swallow too much pool water.

To avoid spreading digestive conditions, don't go swimming if you have a stomach problem. Even if you're feeling fine, though, faecal bacteria can be pretty tenacious. "If you're considerate, you'll shower before you go in the pool," says Seto. It goes without saying that you should also have one immediately afterwards.

Likewise, anyone suffering with a cold should steer well clear of the pool, as should anyone with a skin condition, for their own sake as well as everyone else's. The skin is an effective protective barrier, but not where it's broken, as with cuts, ulcers and abrasions.

"If you have an ulcer with pus coming out of it, or a terrible skin rash, then five parts per million of chlorine can't kill that," says Seto.

"In fact, if you have a rash, the chlorine could worsen it." If someone else in the pool has a rash, he adds, tell the lifeguard - and if the lifeguard doesn't do anything, "I'd take my bag and leave."

Viral infections such as warts are also commonly spread at pools, particularly among children. Here, the disinfecting footbath is helpful, but it's a good idea to wear flip-flops in the changing room and around the side of the pool, rinsing them with water beforehand to make sure they're free from dirt.

While the skin might be totally protective, mucous membranes such as the mouth and eyes aren't. Hung says ear infections can be contracted either via the ear or the nose, which is linked to the middle ear by the Eustachian tubes.

So goggles are a very useful addition to your swimming costume - surprisingly, so is a hairdryer. "The worst thing you can do, which people sometimes do after swimming, is poke your ears with cotton buds," says Hung. "They can contain infections. It's better to blow your ears dry with hot air."

The Leisure and Cultural Services Department (LCSD), which operates Hong Kong's public pools, also asks swimmers to bring only clean clothing and personal belongings into the pool deck area; to refrain from sharing towels and other personal items; and to make sure their towels and swimsuits are completely clean.

If you see anyone wearing anything other than swimwear, talk to whoever's in charge.

Hong Kong's public pools operate to very high standards, with weekly independent lab tests of the level of free residual chlorine, the pH, the total bacterial count, and the presence of and , the bacteria that can cause cholera. The results of all these tests are even displayed on the LCSD website.

In addition, says an LCSD spokesman, "Each public swimming pool is equipped with a filtration and sterilisation system which continuously filters and disinfects the pool water throughout the opening hours of the pool. Pool staff are required to follow stringent operational procedures to maintain pool cleanliness.

"Water tests are carried out on an hourly basis to measure the free residual chlorine level and the pH value, with the aim of ensuring the disinfection capability of the pool water."

The LCSD also monitors the cleanliness of its pools, keeping surface drains clear from stagnant water and dirt, cleaning all pools gutters and sides nightly, and conducting a more thorough cleansing once a week, during which an extra dose of chlorine is added.

For private pools, says Seto, "If you see that there's a lot of dust and dirt in the bottom, talk to the pool owner. Once a pool is dirty with dust, especially organic material, it's much easier for diseases to spread."

For all private pools, he adds, "It's important to make sure that it's licensed by the government. [If so] you can assume, as you can if you go to a public pool, that the conditions are very strict."

So, in Hong Kong at least, you should be safe to swim with confidence. Venture further afield, however, and it's even more important to take all the right precautions.

As Hung points out: "Most of the patients who come to see me with infections resulting from swimming got them when they swam abroad."