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• Low mood or irritable mood most of the time
• Trouble sleeping or sleeping too much
• A big change in appetite, often with weight gain or loss
• Tiredness and lack of energy
• Feelings of worthlessness, self-hate, and guilt
• Difficulty concentrating
• Slow or fast movements
• Lack of activity and avoiding usual activities
• Feeling hopeless or helpless
• Repeated thoughts of death or suicide
• Lack of pleasure in activities you usually enjoy, including sex

There is not just one cause of depression. It is a complex disease that can occur as a result of a multitude of different factors, including biology and emotional and environmental factors. For people biologically vulnerable to depression, it may sometimes start with a significant life event, such as the loss of a loved one or a change in one's life or after being diagnosed with a serious disease. For others, depression may just occur for no apparent "reason." In fact, there does not need to be any apparent "reason" for the symptoms of depression to occur in people who are vulnerable to the illness.

Biochemical factors

Depression is a type of mood disorder that some believe is triggered when neurotransmitters in the brain are out of balance. Neurotransmitters are chemical messengers that help the brain communicate with other parts of the body. These chemicals help regulate many physiological functions.

Low levels of neurotransmitters may play a role in why some people are more susceptible to depression, including the neurotransmitters:

• serotonin

• norepinephrine

• dopamine

Genetic factors

Having an immediate family member with depression or a mood disorder can increase your risk for depression. The American Psychiatric Association (APA) states that if one identical twin is diagnosed with depression, the other twin has a 70 percent chance of developing it.

However, depression can occur in people with no family history, which is why some scientists believe it can be a product of both genes and life experiences.

• Identify ways to improve access to effective strategies by those at risk. For example, preventing intergenerational transmission of depression requires effective, acceptable, and scalable interventions for families and in the school environment, while preventing depression in older people with physical illness requires embedding preventive research in health care settings and social services. To reduce the effect of depression on productivity losses, including premature retirement, prevention programs suitable for the work environmentneeddevelopment. The use of new media,such as e-mental health and smart phone technologies, and the use of lay health counselors may boost dissemination, especially in low- and middle-income countries.
• Study how depression in one person has cascading or contagion effects on others close to him/her in terms of their productivity, health, and well-being.
• Because preventive interventions are likely to have multiple effects beyond depression or even mental illness in general, measure multiple outcomes beyond health, including economic, educational, and social role functioning.
• Focus on risk factors for developing mental disorders in general (eg, sleep disturbance, social isolation, child abuse and neglect, and disabilities associated with medical and neurological disease). In the prodromal or at-risk phases, it is not yet clear which disorder will develop. Such an approach will extend the focus of prevention and stimulate collaborations among different subfields of prevention of mental disorders.
• Develop interventions based on causal mechanisms underlying risk: in addition to social stressors, address psychological and biological markers of risk (neuroticism, sleep, proinflammatory cytokines, etc) to achieve more targeted and rational interventions.
• Incorporate economic analysis of costs and benefits. Development of stepped-care programs will facilitate approaching larger numbers of people at risk with self-help, e-health, or other relatively inexpensive and easy-to-disseminate nonconsumable materials, restricting the use of more intensive interventions to those who do not respond favorably.
• Universal, selective, and indicated prevention may be used in concert. Currently, it remains unknown what is the most effective strategy to reduce the burden of depression; targeted research is needed to determine which type of prevention is associated with the best value for money.
• Develop strategies for producing more enduring preventive effects. Although evidence exists for the efficacy of interventions for reducing the onset of depressive episodes, additional research is needed on how to make these effects endure. The use of booster sessions and Internet technologies should be explored.