Depression is a serious mental illness that affects people of all ages and walks of life. A person with depression may experience a range of often disabling emotional and physical symptoms that can interfere with the ability to function normally in everyday life. While the cause of depression is not fully understood, it is generally believed that a number of issues may influence the presence of depression, ranging from low self-esteem, life events and critical illness, as well as genetic, biological and environmental factors. Researchers believe that people suffering from depression may have imbalances in serotonin and norepinephrine, which are neurotransmitters that enable brain cells to communicate. Decreased levels of serotonin and norepinephrine are also believed to cause the irritability, sleep problems and fatigue associated with depression. Depression also appears to run in some families, suggesting that a family history of depression increases the risk for children and siblings in successive generations. People who lack self-confidence, who tend to be overly critical of themselves, who have a generally negative outlook on life, or are unable to cope well with stressful events are also believed to be at risk for depression. Traumatic life events such as a death, job loss or divorce may trigger depression. Even welcome events like graduating, getting married, having a baby or moving into a dream home may lead to depression, often because regular routines are disrupted and new demands emerge. Depression often occurs in conjunction with certain illnesses, such as cancer, Parkinson's disease, heart disease or Alzheimer's disease. Certain medications - like barbiturates, benzodiazepines, beta-blockers and codeine - are associated with depression as well. A history of physical, emotional or sexual abuse may often be a cause of depression later in life. Many famous people, past and present, have suffered from depression including Vincent Van Gogh, Ernest Hemingway, Michelangelo, Mark Twain and Mozart. More recently, celebrities like comedian Jim Carrey, actor Hugh Laurie and actress Brooke Shields, as well as Harry Potter author J.K. Rowling, have talked openly about their bouts with depression, while actor Owen Wilson's depression was made public following his attempted suicide. While the exact cause of depression is not completely understood, depression can be successfully treated in more than 80 percent of people who seek help. If you - or someone you know - is affected by depression, it's important to consult a mental health professional as soon as possible.