The standard of hygiene in first-class hotels is in general o.k., but you should not drink the tap water (though it can be used for teeth brushing). The better hotels provide bottled drinking water for free. You should be careful with the food from foodstalls or in simple restaurants. Eat only things that have been thoroughly cooked and are served really hot. The large amounts of chili that are part of nearly every Thai dish may have a disinfecting effect, but you should not rely on this! (The people themselves are nearly always neat and clean, but sometimes they use dirty water for dishwashing, etc.) Chiang Mai and Bangkok are malaria-free, but if you plan to do trekking tours in the hills, you should take a prophylactic drug.  Malaria is a serious disease that can be lethal! (You can look up details about malaria at Pre Travel Medicine or at the malaria page of the CDC). Vaccinations against tetanus, hepatitis, and salmonella infections are also advisable.

Safety

Thailand is generally considered a safe country and for tourists, Chiang Mai and the North of Thailand are very safe places with no open violence in the streets. Of course there is criminality, but it is unlikely that you will encounter it. The worst that could happen to you is probably that you get pickpocketed or that a tout tries to take you to a shopping center - very often these people will tell you that a temple or museum that you are about to visit is closed - and instead they suggest a visit to a jewellery or shopping mall . Just ignore them. If the temple happens to be really closed, you can come back and get a taxi then. Always refuse when somebody offers you a free tour at tourist attractions.

As everywhere, of course, it is better to avoid carrying large amounts of cash; credit cards or traveller`s cheques are preferable.