From NHS Site

Upset tummies

One in five tourists on package holidays fall ill, most with stomach upsets. Tummy bugs or gastrointestinal illnesses are the most commonly reported travel-associated infection.

Healthy eating tips:

Wash your hands thoroughly before eating food.
Wash your hands with soap and hot water after using the toilet.
Drink water from sealed bottles. Hot tea and coffee, fizzy drinks, beer and wine should be OK too.
Avoid local dairy products, particularly ice cream and cheese, which can cause diarrhoea.
Fish and shellfish may be hazardous even if well cooked. Ask a local what’s safe to eat.
Eat only cooked vegetables and avoid salads, which will probably have been washed in tap water, or not at all.
Avoid freshly cut fruit and peel all fruit.
Avoid food that has flies buzzing round it.

STIs

Holiday excesses can lead to irresponsible behaviour, including unsafe sex. Many STIs, including chlamydia, have no noticeable symptoms.

Condom advice:

Buy a pack of condoms with the European kite mark, which shows they've been tested.
Condoms can be damaged by contact with oil-based products such as suntan lotion, baby oil and even lipstick.
Condoms can also be damaged by the heat so keep them in a cool, dry place.

Sunburn

More than a quarter of adults think getting burned is all part of getting a tan. Binge tanning is the quickest way to get skin cancer.

Tanning tips:

Avoid the sun between 11am and 3pm.
Take care never to burn.
Cover up with a t-shirt, wide-brimmed hat and sunglasses.
Use factor 15+ sunscreen and re-apply regularly.
Use factor 30+ sunscreen for children.

Malaria

Malaria, which is spread by mosquitoes, is most common in Africa, the Indian subcontinent and south-east Asia. It strikes over 2,000 British travellers a year.

Anti-malaria tips:

Take anti-malarial tablets before you travel.
Don't worry about the possible side effects: having malaria is far worse.
Use a mosquito net and strong insect repellent, and cover up bare skin from dusk to dawn.
If you think you’re showing symptoms of malaria, get medical help immediately.
Alcohol

Alcohol is generally cheaper abroad, and this can mean it's tempting to drink more. Alcohol and sun can be a very bad combination because alcohol dehydrates the body.

Safe drinking tips:

Stay hydrated: drink lots of water.
Stick to the recommended daily unit limits.
Avoid drinking alcohol in the full glare of the sun.
Remember that most accidents abroad are alcohol-related.
Unsafe sex is often blamed on too much alcohol: use a condom.