A major aspect of communicable disease control relates to breaking the chain of transmission" or interruption of transmission. This may mean changing some components of man's environment to prevent the infective agent from a 'patient or carrier from entering the body of susceptible person. For example, water can be a medium for the transmission of many diseases such as typhoid, dysentery, hepatitis A, cholera and gastroenteritis. Water treatment will eliminate these diseases. Depending upon the level of pollution, this may vary from simple chlorination to complex treatment. However control of the source of contamination is an important long-term measure. Food-borne disease is particularly prevalent in areas' having low standards of sanitation. Clean practices such as hand-washing, adequate cooking, prompt refrigeration of prepared foods and withdrawal of contaminated foods will prevent most food-borne illnesses. When the disease is vector-borne, control measures should be directed primarily at the vector and its breeding places. Vector control also includes destruction of stray dogs, control of cattle, pets and other animals to minimize spread of infection among them, and from them to man. On the other hand, episodes of infection either by droplet or droplet nuclei are not usually controlled effectively by attempting to interrupt their mode of spread; reliance is placed on early diagnosis and treatment of patients, personal hygiene and proper handling of secretions and excretions. In short, blocking the routes of transmission imply an attack on environmental factors, that is, to bring about an adjusted equilibrium between host and environment through encouraging some ecological influences and inhibiting others.