For your security, Thailand is as safe as any other country. So don't be scared, tourism is the main income in Thailand and Thai authorities do not want to lose you.

Concerning the heathy conditions, it's different, tourists can become sick like many other countries in the world and before going to Koh Chang, be sure that you have all the needed injections as well as having good travelling insurance. In case of an accident, it will save you a lot of trouble and money; costs in private hospitals are quite high, especially if you need to spend a few days there or need to return to your home in an emergency.

The 4 biggest risks in Koh Chang are motorcycles accidents, sunburns, mosquitoes, and food poisoning:

• If you decide to rent a motorcycle, be very safe, remember that you are not home and even on Paradise island a helmet will protect you. If you don't feel comfortable on a bike, then take a taxi, it would be stupid to shorten your holidays and go back home in an ambulance.
  And in case of an accident, there is the Koh Chang International Clinic located in White Sand beach.
  
  
• Cloudy or very sunny, if you are planning to lie down at the pool or at the beach, use sun lotion. Without it, you will burn in a few minutes and the following days will just be pains.
  You can find different brands and levels of sun lotions in all drugstores as well as V-mart and 7/11 shops. In the evening rehydrate your skin, you can find natural Aloe-vera gel or Coconut oil in a lot of stores. They are cheap but very helpful!
  

• This area is marked as Malaria and Dengue Fever risks even if there haven't been real big cases. The best way is to avoid mosquito bites by using repellents.
  You can find good repellents in all drugstores but also V-Mart, 7/11, and Tesco shops. When you are preparing yourself for the dinner, just apply some repellent on your skin (legs, feet, arms, and neck).
  
  
  
• You can get food poisoning anywhere in the world, not only in Thailand or Asia. The risk Zero does not exist. It can come from what you will eat but also what you will rink or maybe touch.
  Don't become paranoiac by starting to disinfect all that you are touching and drinking only boiled water and eating sterilised food. There aren't so many people who get food poisoning but it can happen and you must be prepared and know what to do.
  If you start to have diarrheas, you are vomiting and get a fever, don't lose time and ask to be brought to Koh Chang Clinic. They will do the right test and give you the medicines you need. 
  Koh Chang Clinic is very clean, the nurses and doctors are very good and they have very modern equipment. It's at this moment you will enjoy having travel insurance. You will need to pay first, in Thai baht or with your credit card. They will give you all the paper for your travelling insurance which will pay you back.
  To give you an idea, the diagnosis by the doctor, a blood test, a couple of injections of medicines, and the prescription of new medicines can cost between 8,000 and 10,000 bahts. 

If you start to feel sick, do not wait that it becomes worse, go directly to a drugstore and describe your symptoms. They will give you medicines and if after a couple of days it's still the same, ask to see a doctor.
There are a lot of drugstores along the island.

There is a very good dental clinic in Koh Chang, very modern with excellent dentists. Depending on where you are coming from, they can be much cheaper.

Like the drugstores, there are also a lot of optical shops. They don't sell copies like street stalls, only genuine products. All of them are proposing good discounts if you buy one, two or more pairs. They can also check your eyes and make the glasses you need.
Contact lenses are also very interesting. They normally sell DUNA Life which is manufactured by Bausch & Lomb Ireland.
Some of them will offer you refreshments while you are trying the different models.