​Chronic diseases

A) ​develop and persist over a period of time.
B) ​are due to infectious agents such as bacteria or viruses.
C) ​are not as common today as during the 19th century.
D) ​include influenza and pneumonia.

Chronic diseases are long-term conditions that typically develop slowly and persist over a lengthy period of time. They include conditions such as diabetes, heart disease, and cancer, among others. They are not typically caused by infectious agents like bacteria or viruses, and they are much more prevalent today than they were during the 19th century. Influenza and pneumonia, while serious illnesses, are not considered chronic diseases.