Depression is a condition typically characterized by sad moods and loss of interest. Feeling sad and low is very common amongst people, but these feelings only qualify as diagnosable clinical depression when they pass a certain threshold characterised by persistent feelings of sadness and general low moods. This illness is commonly assumed to be caused only by external factors, but there are several biological factors that contribute to and manipulate this condition, including: neurotransmitters, neuroplasticity and endocrinology, genetics, and psychoneuroimmunology. These contribute to the condition, but none can be singularly distinguished to be the sole causing factor of depression.