Health is defined by WHO as 'a state of complete physical,social and mental well-being and not merely the absence of disease or infirmity'.With the variation of natural environment and the progress of human society,due to the continuously change of mortality factors and disease clusters,human beings have been confronted with,and still will face to such health risks as:the prevalence of some endemics unable to be completely controlled;the bouncing back of some old infectious diseases ever being controlled;the spread of new infectious diseases,the health threat of harmful chemical substance;the ...
