Malaria is an illness which you may catch when visiting tropical areas of the world. It is caused by type of parasite which is carried in certain breeds of mosquito. Although it is generally not too dangerous, if left untreated it can be life-threatening. There is no widely available vaccine against malaria, and there is no method of prevention that is 100% effective. If you are going to a country which is high-risk for malaria (you can check this on the Fit For Travel website) you should take preventative measures both before you go and whilst you're there.
You should use a two pronged approach to protect yourself: both taking antimalarials, and using anti-insect sprays, mosquito nets, and making sure all windows and doors shut at nighttime when you are there. Your doctor will be able to advise you on the best anti-malaria tablets to take based on where you are travelling to. When she know what you are looking for you can buy these tablets online. You should begin treatment in a few days before you go away and not finish it until you have been back for a few days.