Q.What are acquired diseases?

Health, per the WHO definition, is a state of complete physical, mental and social well-being, not merely the absence of disease or infirmity — it reflects how efficiently an organism's body and metabolism are functioning, and is safeguarded by hygiene: a balanced diet, clean water, personal/community cleanliness, exercise, disease awareness, proper waste disposal and vector control.

Disease, correspondingly, is disturbed or deranged functioning of one or more organs/organ systems, caused by infection, a defective diet, or heredity. Diseases are classified in two steps. First, congenital diseases are present from birth (genetic or metabolic in origin, historically incurable, though gene therapy and enzyme-replacement therapy can now treat some), versus acquired diseases, which develop after birth. Acquired diseases are then split into communicable (infectious) diseases — transmitted from an infected person to a healthy one, directly or indirectly, caused by pathogens such as viruses, bacteria, fungi and helminth worms — and non-communicable (non-infectious) diseases, which cannot be transmitted person to person, such as cancer and deficiency diseases.

A recurring concept across the chapter's infectious diseases is the incubation period — the interval between a pathogen's invasion of the body and the appearance of the disease's clinical signs.