Clinical depression is very common. Over nine million Americans are diagnosed with clinical depression at some point in their lives. Many more people suffer from clinical depression because they do not seek treatment. They may feel that depression is a personal weakness or try to deal with their symptoms on their own. On the other hand, some people feel comfortable admitting their symptoms and seeking help. This discrepancy may explain the differences in reported cases of depression between men and women, which indicate that more than double the number of women than men are clinically depressed. According to the number of reported cases of depression, 25% of women and 10% of men will have one or more episodes of clinical depression in their lifetime. The biological basis of clinical depression originates in the brain. The brain is made up of a complex network of nerve cells, called neurons, and brain chemicals, called neurotransmitters. Neurotransmitters transmit messages from one neuron to another. Two of these neurotransmitters are not produced in sufficient quantities in the brain of a depressed person. Due to this deficiency, too few messages are transmitted between neurons and symptoms of depression occur. In clinical depression the chemicals in the brain are out of balance. New technology allows researchers to take photos of the brain that show activity levels in the brain. These imaging techniques like f-MRI and PET actually create images of how active different parts of the brain are. Some studies using these types of techniques have suggested that the activation patterns in the brains of depressed people are different from those who are not. These tests can help doctors and researchers learn more about depression and other mental illnesses. Because this research is fairly new, it is not yet used to diagnose clinical depression. Strong evidence suggests that clinical depression runs in families. However, just because a person has family members with clinical depression does not guarantee that they will develop it. Likewise, you may get it even if no one else in your family has had it. People with high levels of stress are much more likely to develop depression than those who don't. Although most people go through a period of "down" after a particularly stressful event such as divorce or the death of a loved one, sometimes extremely difficult stressors can trigger depressive episodes in some people who are susceptible to the disease..