If a real health emergency arises, evacuation to Bangkok is the best option.
Cambodia is safer than many people realize, but you still need to exercise caution. Most violence occurs against Cambodians. However, tourists have been mugged and sometimes killed in Phnom Penh and on the beaches of Sihanoukville. Keep most of your cash, valuables, and your passport in a hotel safe, and avoid walking on side streets after dark. Siem Reap has much less crime than the capital, but that's starting to change. Avoid motos late at night. Moto theft is one of Cambodia's most widespread crimes, and the unfortunate driver often winds up dead.
Land mines laid during the civil war have been removed from most major tourist destinations. Unexploded ordnance is a concern, however, around off-the-beaten-track temples, where you should only travel with a knowledgeable guide. As a general rule, never walk in uncharted territory in Cambodia, unless you know it's safe.
Cambodia has one of Asia's most atrocious road records. Accidents are common in the chaotic traffic of Phnom Penh and on the highways, where people drive like maniacs. The better the road, the scarier the driving. Unfortunately, chauffeurs are some of the worst offenders. Wear a seatbelt if they're available, and if you rent a moto, wear a helmet.