Rabies
While you probably won't come across this problem in the city, out in the countryside and around outlying tourist attractions like the ruined temples of Angkor Wat, wild animals are free to roam. The chances of being bitten by a rabid dog or bat are actually pretty slim, but taking care when you're exploring can help keep you safe.
If you are unfortunate enough to be bitten, then wash the wound immediately with clean water and head for the nearest hospital. Of course, there are vaccinations you can get before you go, and if you're intending spending a lot of time off the beaten path, you might want to check out your doctor's advice on which shots you'll need to combat this unpleasant condition.