What's the best way to stop my children getting jet lag?

It depends on the age of the children. If they are younger than four or five, there's nothing you can do as they are a law unto themselves on a flight, and no amount of cajoling will get them to sleep when you want. Over this age, there are possibilities.

The key to the prevention is to immediately set your watch to the destination time. You then work back to what you would normally be doing with your children at that time and try to get them to do it. If they should be asleep, then use a teaspoon of Piriton syrup. It's an antihistamine with a mild sedative side effect.

However, if all does not go to plan and you arrive in the daytime with your body still wishing it were in bed, forget all the medication available and stay awake for as long as possible: hit the pool, go for a refreshing walk. Then eventually try to get to bed and set the alarm for a late start. After this, all should be fine. Most take only a day to adapt.

How do I get my children to equalise their ears on a flight?

This is a parents' nightmare. Aware of the sleepy couple behind them, they fly in fear of the little darling letting rip with a 100-decibel shriek during the flight.

Often, this is caused by the child's ears hurting on ascent or descent. As the plane goes up, the air gets thinner, expands and needs to escape from the middle ear. It comes out of the Eustachian tube, which connects the middle ear to the back of the throat.

The best way to assist this is to get your children to wiggle their jaws on ascent. Give them a sweet or a bottle to suck on, or breast feed as the plane goes up to cruising altitude.

It can be a tad harder on the way down though. Here the air is contracting. To stop the pain from developing in the sinuses and ears, you need the child to push more air into these air-filled spaces. It's easy for adults - just make the motion of stopping a sneeze midway. But try explaining this to a child. A good way to mimic this is to give them a balloon: any child old enough to blow one up will effectively equalise the pressure on descent. If they are too young, place a hand very briefly over their mouth and nose as they exhale. This will create enough positive pressure to pop their ears.

Is there a drug that we can take for altitude sickness?

Diamox (the generic name is acetazolamide) is usually diagnosed for people who suffer from glaucoma - where the pressure in the eye increases - or in situations where the lungs become waterlogged with fluid known as oedema. The action of the drug helps reduce the fluid build-up, so the lungs can breath normally. This is why it has been used to combat acute mountain sickness (AMS).

Because the level of oxygen is much lower the higher we go, our bodies respond by initially breathing faster and deeper, but then the red cell numbers also increase to try to carry more oxygen to the tissues. Diamox helps by increasing our rate of respiration, the effectivity of the oxygenation of the tissues, and prevents the fluid from accumulating in our lungs.

AMS only really occurs above 2,000m, and about 50% of trekkers will get it above 3,500m. However, one in 20 trekkers will get severe, life-threatening AMS above this level, so you have to be careful. The more rapidly you ascend, the more likely you are to get it. The symptoms are: headache, nausea, vomiting and loss of appetite.

Get some Diamox for your trip but only use it in certain situations, such as after you have attempted to acclimatise properly, which involves spending at least three days at the height you have arrived at. You need a high carbohydrate, low red-meat diet. Avoid cigarettes and alcohol, and remember not to over exert yourself. If, despite all this, you still get symptoms of AMS, you should descend and stay at a lower level; or use Diamox and do not go any higher until you have reacclimatised and feel well.

Finally, never increase your sleeping height by more than 300m a day above 3,500m, and if you do start to get symptoms of severe AMS, such as a persistent cough or frothy pink sputum, then take oxygen and descend as soon as you can to seek medical attention.

When should we get our shots before we go away?

In a perfect world, you should begin any course of shots six weeks before you go. Hepatitis A should really be given two weeks before you go, but it's fairly effective even sooner before travel. The same is true of typhoid. Tetanus and polio are normally boosters for childhood shots so can be administered even on the day of travel.

The one that really matters is rabies. If you are venturing into the wilderness and there is a rabies risk, then you have to have the vaccination. This is a three-shot course and needs at least a week in between each shot.

Most good GP practices have a nurse who can administer the travel shots, so phone up and find out what you need for your destination. Then, make sure that they have it in stock. You should not be charged for tetanus and polio, and some surgeries even give the Hep-A shot for free. The rest you will have to pay for.

Can we drink the water?

I would only drink the water in the United States, Canada, Australia, New Zealand and western European countries. While it may be "safe" in other places, I don't think it is worth the risk of gastric upsets, which can ruin a holiday, for the sake of a cheap bottle of water.

If you are somewhere where it is obvious you can't drink the water, and bottled water is expensive or hard to come by, there are now cheap portable water-filter kits that can provide many litres of drinking water. Likewise, the old-style iodine or Milton are effective, but they do make the water taste awful. You could stock up on lemons or limes to squeeze into the sterilised water, to give it that palatable restaurant taste!

We would love to go to Australia but are worried about the spiders. How dangerous are they?

There are venomous spiders in Australia, and they do have the potential to kill. But Australia is visited by millions of people and they have a wonderful time without so much as seeing a spider, let alone getting bitten by one of the toxic varieties.

The one to look out for is the redback spider. This is a close relative of the black widow that is found on the American continent. It is not an aggressive spider but contact comes by accident when the unfortunate victim either sits or lays a hand on it. Redbacks are known to live under the seats of lavatories in the Outback, so it is advisable to always check before you sit down. The consequences of a bite can be a trip to hospital for a shot of anti-venom.

The really nasty funnel-web spider lives in the suburbs of cities such as Sydney and can bite you without provocation. It usually lives under raised houses in its funnel-shaped web and can be deadly. It is important to lay the victim flat, apply a pressure bandage to the affected area, but not a tourniquet, and make sure that they are transported to a medical centre as quickly as possible.

We are planning a trip to Lake Malawi but have been warned about bilharzia. What is it?

Bilharzia, or schistosomiasis as it is known medically, is an illness caused by a little worm, and there have been reports of cases at Lake Malawi. Like a lot of parasites, it needs a host to lay its eggs in, such as humans.

Infection can occur when walking barefoot or swimming close to the lakeside, as the worm can penetrate the skin and lay its eggs in the liver, bladder or blood supply to both these organs.

The eggs are then excreted by the human host into freshwater, where they hatch and infest the intermediate host, a freshwater snail that lives in the reeds and grasses beside African lakes. The larva develop and then re-emerge to reinfect man, again by penetrating the skin. Symptoms can range from feeling mildly unwell, through to passing blood on urination or even liver failure in the later stages.

But before you worry too much, it is easy to diagnose and easy to treat, too. If you feel unwell after a couple of weeks, then go to a doctor who will be able to test your blood - it is possible to pick up the infection before the eggs have been laid - and the treatment involves a day's worth of tablets called praziquantel.

To prevent the worm from coming into contact with you, either swim farther out in the lake, or wear protective shoes if you are wading through the reeds.

What is the best way of treating diarrhoea?

There's evidence to show that more than 75% of travellers will get some illness when they are away, the main one being the big D. Simple measures to combat infection involve washing all your food, then peeling it, and if possible drinking only bottled water.

A single 750mg dose of the antibiotic ciprofloxacin has been shown to be effective in 80% of cases if taken within a day or so. If I am in a part of the world where a long bus journey is needed, I never travel without this drug. It is available only on prescription, so ask your GP. He or she can only issue it on a private basis as it's to be taken abroad. Expect to pay about £5 a dose.

I am prone to bladder infections. Is there a way I can stop them from occurring when I go on safari?

You suffer from cystitis, which is where bacteria that normally inhabit your lower gastrointestinal tract can find their way to your bladder and multiply, causing pain on urination, increased frequency of urination, and often an odd odour to your urine. Women get it far more frequently than men. To prevent it occurring in the first place, always drink plenty of fluids, especially cranberry juice.

If things don't improve after taking over-the-counter remedies, your doctor can prescribe antibiotics. But on safari you may be a long way from medical help and, if left untreated, the real problems can set in. It is possible to contract full-on kidney infection, which would lead to severe fever, vomiting and pain in your lower back on the side that is infected. This, in turn, can sometimes lead to dehydration, delirium and hospitalisation.

Take your usual remedies with you, and if you get symptoms, try these for a day. If there is no improvement, you need to increase your fluid consumption to about five litres a day and start a course of antibiotics such as trimethoprim (200mg twice a day for three days) or cephalexin (250mg twice a day for the same period).

The strongest and widest spectrum antibiotic is called ciprofloxacin, which is taken at a dose of 100mg twice daily, again for three days.

If you are going abroad for a long period, I recommend you get hold of two courses of either trimethoprim or cephalexin, and one course of ciprofloxacin.

Can I go on a diving holiday if I am pregnant?

No. The same is true if you are trying to get pregnant. One of the issues with diving is that for all the tables and dive computer algorithms we have, there is always a chance, however remote, that you could get the bends. This would have health implications for the unborn child.

Nitrogen bubbles are easily absorbed by neuronal or nerve tissue. If a pregnant diver gets a bend but it stays undiagnosed, as it often does, then the foetus could easily suffer decompression sickness (DCS), or a nerve bend.

I know that a lot of this is speculation, as there is not much research on the matter, so I always say: why take the risk when you can put off a diving holiday until after the baby is born?

· Dr Jules Eden created E-Med as an online support service for anyone who finds themselves a long way from a doctor's surgery. You can get free pre-travel information on any destination in the world. Log on and click on the Travel Clinic icon.