As infectious diseases have been brought increasingly under control, more people live into old age, when they become susceptible to long-term illnesses. As a result, chronic diseases now comprise a large proportion of health-care systems' total caseloads.
但是，随着时间的推移，发达国家慢性病发病率几乎确定地增长着。随着传染病日益得到控制，更多的人能够活到老年，而老年人更容易得长期疾病。因此，如今，慢性病构成了医疗体系患病总数的最大头。