凡疾病的发生或慢性疾患的恶化有明显的周期性气候特征的，称为“季节病”。季节病中除少数疾病如中暑、痢疾等发病在夏季外，多数季节病大都发生在冬半年，如慢性气管炎、肺心病、哮喘、心肌梗塞、肺炎、感冒、精神病等。有些疾病在隆冬季节为发病高峰期，这与持续低温有关。有的则有秋末冬初和春季两个发病峰期。在秋末冬初和春季是大气环流大规模调整和转换的时期，天气变化比较急剧，冷空气活动频繁，致使疾病多发。
季节性情感障碍和产后抑郁症曾经也面临过同样的情况，但是，现在它们已经被正式收录到《精神病诊断与统计手册》中了。
However, this was also the case with seasonal affective disorder and postpartum depression, which are now formally recognized in the Diagnostic and Statistical Manual of Mental Disorders.
南半球许多国家正处于冬季流感季节，因而将重点特别放在了南半球的流行病学现状方面。
Particular emphasis was placed on the epidemiological situation in the southern hemisphere, where many countries are experiencing their winter influenza season.
在季节性流感和以前的一些大流行病当中，怀孕已经是一个尽人皆知的风险因素了。
Pregnancy is already a well-documented risk factor in seasonal influenza and in previous pandemics.
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