Clean drinking water is paramount, but you can't always trust even bottled water. A charity worker once told me about counterfeit water bottles, and since that time I've always tried to avoid crown caps and only buy bottles with sealed plastic caps.
Malaria is common in the tropics and particularly in Equatorial Africa. I recommend prophylactic medication for anyone travelling to these areas. These days you can choose from many different products.
You should always use effective protection against mosquitoes, as they spread many other diseases too. One of the worst is dengue fever, which has recently become more common. There is no medication or vaccine for it, and roughly a hundred Finns contract it every year.
Netting around the bed is just as important as medicines, and in malaria-ridden areas the nets are often treated with mosquito repellents.
Don't touch stray dogs!
Rabies is a dangerous disease transmitted through animal bites. The vaccine should not be taken as a precaution, but if you are bitten by an animal, you should visit a doctor right away. The set of vaccines provides you protection even after exposure.
Infectious diseases regularly break out around the world, and most of us have heard abbreviations like SARS and MERS. Careful hand hygiene is essential, and why not wear a mask that covers your mouth and nose in the aeroplane or other vehicles, if you feel you would benefit from it?
The risk of a typical traveller catching a dangerous infectious disease remains very slight.
For more travel-related health information, visit the website of the National Institute for Health and Welfare (THL) if you are in Finland. Areas considered risky for travellers include Sub-Saharan Africa, the Indian subcontinent, Southeast Asia, and Central and South America.