Adequate hygiene require an adequate and convenient supply of clean water. In much of the developing world maintaining an acceptable level of cleanliness is difficult or impossible for much of the population due to lack of adequate water supply and sanitation. This results in the rapid spread and extent of diseases such as trachoma which are rare in the developed world.
Outward signs of good hygiene include the absence of visible dirt (including dust and stains on clothing) or of bad smells. Since the development of the germ theory of disease, hygiene has come to mean any practice leading to the absence of harmful levels of bacteria.
Good hygiene is an aid to health, beauty, comfort, and social interactions. It directly aids in disease prevention and/or disease isolation. (That is, good hygiene will help keep one healthy and thus avoid illness. If one is ill, good hygiene can reduce one's contagiousness to others.)
Washing (with water) is the most common example of hygienic behavior. Washing is often done with soap or detergent which helps to remove oils and to break up dirt particles so they may be washed away.
Hygienic practices -- such as frequent hand washing or the use of boiled (and thus sterilized) water in medical operations -- have a profound impact on reducing the spread of disease. This is because they kill or remove disease-causing microbes (germs) in the immediate surroundings. For instance, washing one's hands after using the toilet and before handling food reduces the chance of spreading E. coli bacteria and Hepatitis A, both of which are spread from faecal contamination of food.