Before answering your question i would like to say that in accordance with the World Health Organization (WHO), depression is all set to become the second most fatal disease by 2020 with only coronary heart problems preceding it. A lot of people just like you wonder why this condition is given so much of attention when it is just a mental illness. But yes these mental illnesses, depression for example can kill you. How?
Diseases like cancer and tuberculosis can kill you but depression cannot kill you by itself. It has been proven that victims of this condition have biochemical and endocrine abnormalities in their brain's, but this per se is not life threatening. Statistics have shown that 90 percent of the people who have died of suicide have had clinical depression in the past. Patients with suicidal tendencies usually suffer from depression coupled with other problems such as chronic alcoholism or substance abuse. if depression over a prolonged period of time is left unattended to or ignored can lead to several complications, complications which can be life threatening. Just as our muscles require nutrition to grow, our brain has circuits which are dependent on substances called neurotransmitters. These neurotransmitters regulate the electrical impulses in the different parts of our brain.
In depression, there is an imbalance in the production of certain neurotransmitters thereby leading to the malfunctioning of some circuits. The group of muscles which we use the most often usually becomes the strongest and the group which we use the least becomes weak, a similar concept can be applied here. Those circuits which are involved in melancholy and sadness are stimulated repeatedly and those circuits controlled by serotonin (neurotransmitter involved in producing a sensation of happiness) become weak and over a period of time , stop functioning, hence these individuals are in a prolonged state of sadness and they lose motivation to live. This is the reason as to why victims of depression have such strong suicidal tendencies, tendencies which can not be overcome by the inhibitory circuits of the brain.
Thinking that suicide is a coward's way out for depressed individuals is like saying a person who suffered from cancer did not fight hard enough.
Depressed individuals also have a decreased appetite and loss of interest in life, these individuals consume an insufficient amount of calories on a daily basis for a very long period of time, long enough to kill due to malnourishment. Victims of this condition are also prone to other diseases and they often do not pay enough heed towards these illnesses, quite often which can be life threatening.
Hence the seriousness of this condition is often underplayed and it is of utmost importance that if you do have any of the symptoms you must consult a psychiatrist.