Hygiene standards are high in Japan, and medical facilities (although expensive) are widely available. Probably the most important preventative measure is to make sure that you are healthy before you start traveling. Dental treatment is expensive as well, so it's a good idea to have a check-up before you leave. Make sure that you bring extra contact lenses or glasses as well as any medications you might need. Getting correct medications may be difficult - and illegal if you don't have your prescription handy (it’s also a good idea to know the generic name of the drug as well as the brand name). However, medical treatment and drugs are of high quality. No immunizations or health certificates are necessary for Japan. Food is almost always safe to eat and you can drink tap water throughout Japan but should avoid drinking directly from streams or rivers. You should also avoid wading barefoot in rice paddies or stagnant water as there could be parasites living in the water.