The answer is that they are classified into two cqategories; i.e Chronic andf Acute diseases
Chronic diseases are diseases that are persistent or otherwise long lasting. The term chronic is usually applied when the course of the disease lasts for more than three months. They develop gradually out of habitual diet, posture or other condition. They continue beyond expected period of recovery. Examples are; Asthma, Heart diseases, kidney disease, frequent migraines e.t.c
Acute diseases are diseases with rapid onset/or a short course. These diseases start suddenly as reaction to an injury or something else. Examples are; burns, heartburn, strep throat e.t.c