No vaccinations are required for entry to Cambodia, but the CDC recommends that travelers be up to date on routine immunizations and consider getting vaccinated against hepatitis A and typhoid before going. If you plan to spend time in rural areas, rabies vaccination is worth discussing with a doctor.
Malaria is present in forested and rural areas, particularly in the north and northeast. Phnom Penh and Siem Reap are not considered malaria zones. If you're traveling beyond the main cities, take precautions: use repellent (DEET is effective), cover skin at dusk and dawn, and consider antimalarials if visiting high-risk forested areas.
Dengue fever is a more widespread risk than malaria and is present throughout the country including urban areas. The mosquitoes that carry it bite during the day. Use repellent consistently.
Water: Do not drink tap water anywhere in Cambodia. Bottled water is cheap and universally available. Use it for brushing teeth as well. Avoid ice from uncertain sources, though ice in restaurants and cafes in tourist areas is generally made from treated water.
Food safety: Street food from busy stalls — where the turnover is high and food is cooked in front of you — is generally safe. Be more cautious with pre-cooked food that's been sitting out.
Medical facilities: Phnom Penh and Siem Reap have hospitals and clinics that can handle most standard problems; Royal Phnom Penh Hospital and Calmette Hospital are the main facilities in the capital. Outside these cities, medical care is very basic. For serious illness or injury, medical evacuation to Bangkok is the usual recourse — this is why travel insurance with evacuation cover is not optional. Evacuation can cost US$50,000 or more without it.
Bring enough of any prescription medication for your entire trip, in original packaging, with a doctor's letter if the medication might attract scrutiny.