Depression is the most common mental illness affecting 300mn people worldwide. It is a major cause of disability with an enormous global burden of disease. Besides, it is the most frequently encountered psychiatric disorder in primary care. Patients suffering from this serious illness have a poor quality of life and experience significant disruption in social and occupational functioning. Furthermore, such patients are at an increased risk of physical health problems, death and suicide. Depression is very costly both in terms of loss of productivity due to this disease and money spent on its treatment.
In order to diagnose depression or major depressive disorder, clinicians commonly use the Diagnostic and Statistical Manual of Mental Disorders 5th edition (DSM-5) criteria. According to these criteria, depression is diagnosed when a patient reports a two-week history low mood or loss of interest in daily activities plus at least four symptoms from a list which includes weight changes, changes in motor activity, sleep problems, weakness, guilt, difficulty concentrating, and recurrent thoughts of death. Many patients do not present with these classic symptoms of depression, making diagnosis difficult. 
Depression has varied psychiatric manifestations. Some patients, especially children, may not report any changes in mood. Instead they may present with irritability or grumpiness. Forgetfulness and lack of focus is the main concern for some patients. Excessive worrying or anxiety may predominate in the history of some depressed patients. A group of these patients are overly concerned their physical health and may fear having a major illness. At times, some patients may even see depression as a personal deficiency rather than an illness. In some severe cases, the guilt experienced by the patient becomes delusional and the patient believes that they deserve to suffer as a punishment. Depression in such patients can be detected only through proper screening. Primary care doctors ought to refer such patients to specialists if in doubt. 
Depression, a notorious masquerader, becomes especially difficult to diagnose when a patient with the illness complains of physical problems only. Interestingly, two-thirds of patients with depression visiting a primary care physician report physical symptoms only and no depressive symptoms. Depression can mimic many physical illnesses producing myriad presentations. However, most commonly patients suffering from the illness report pain symptoms. The pain is often described as ‘generalised body aches’ but can be localised to one region such as headache or abdominal discomfort. Many patients with depression have symptoms of indigestion and stomach upset. Some patients have changing symptoms involving different regions of the body at different times. Depression in the elderly is more likely to present with physical symptoms. 
Awareness about depression and its protean manifestations is crucial both for physicians and the general public. Depression is a serious health condition and its detection is often difficult. Luckily, however, it is easy to treat. Currently, there are various pharmacological and non-pharmacological therapies available with excellent safety profiles and efficacy. Fortunately, both physical and psychological symptoms associated with depression are highly amenable to therapy. Under-recognition of depression must be minimised so that more and more people can have access to its safe and effective treatment.