Believe it or not, exotic tropical diseases are not your biggest worry in Cambodia. The most common problems are diarrhoea, sunstroke, and dehydration. As for tropical diseases, malaria is the chief concern. Phnom Penh is malaria free. Angkor is malarial though more remote towns present a greater risk. Dengue fever during the rainy season can be fairly common. Both malaria and dengue can be prevented by covering up against mosquitoes and using insect repellent.

Before you go, make sure you are up to date with tetanus, diphtheria, typhoid, hepatitis, and polio inoculations. 

Visit a travel clinic before you leave your home country.