Medical facilities in Cambodia are not all up to international standards. There is one private hospital that lives up to Western standards, the Royal Angkor International Hospital on Airport road, which also features an ambulance service and employs staff that can speak English and other foreign languages. There are a handful of pharmacies around the city, including one near Psar Chas, which sell the basics - so if you need a regular prescription sort out the necessary supplies before departure.
In terms of infectious diseases, mosquito-borne infections are the most common, especially in the wet season. Make sure to take malaria prevention medicine prior to departure following consultation with a prescribing doctor if you are going to the more rural areas, and check what other vaccinations or preventative drugs may be recommended. Powerful insect repellent and large doses of sunscreen should be considered obligatory travel items for Siem Reap.
Indeed, mosquito bites are the most common minor affliction to affect visitors to the city; so too minor stomach upsets and diarrhoea. Never drink the tap water in Siem Reap and make sure to buy sealed bottles of water.