Leisure sickness is one of those modem conditions. How do you know if you have been struck down with leisure sickness Were you struck down by some mystery illness just before setting off on a two-week holiday If it is really the case, chances are you have leisure sickness. A Dutch psychologist (荷兰心学家) identified the condition. He started researching the subject after he became curious about his own pattern of illness. "If I am ill, then it’s at the weekend or at Christmas. Then I heard stories from friends who all said the same thing." He surveyed around 2 000 men and women and estimated that about 3% of the population may suffer from weekend or vacation sickness.
In a further in-depth study of 114 Dutch sufferers, he found that the most frequently reported symptoms included headaches, fatigue, muscular (肌肉) pain and, particularly in relation to vacation. Most of them had been suffering from leisure sickness for over 10 years and associated the beginning of the illness with a major life , such as marriage, birth of a first child or a change of job. Many shared certain characteristics--a high workload, perfectionism (追求完美), eagerness to achieve and an over- developed sense of responsibility, it difficult for them to switch off outside of work. According to him, there are a number of possible explanations for the condition. "One possibility is a kind of competition between information from the outside world -- external information -- and information from the body-internal information."Leisure sickness is regarded as one of those（）

A．modem diseases
B．poor environment
C．state of existence
D．state of general health