RANCHO MIRAGE, Calif. -

As we've come to learn, Robin Williams was battling severe depression, and recently checked himself into rehab. He'd had a long history of drug and alcohol abuse.

He talked about his addiction in an interview with Diane Sawyer in 2006.

"It's just there.. lays in wait for the time when you think, it's fine now, 'I'm okay,' and then, the next thing you know, it's not okay," Williams said.

According to the National Institute of Mental Health, major depression is one of the most common mental disorders in the U.S. But despite how common it may be, there's often a misconception about what depression really is, and just how serious it can be.

In 2012, an estimated 16 million people suffered from a depressive episode or depressive symptoms. It's commonly mistaken as being the same thing as sadness.. but Eric Woodard, a psychiatrist at Eisenhower Medical Center says depression is very much an illness.
     
"Very much like diabetes or high blood pressure, or high cholesterol that is both environmentally induced or influenced, as well as your genetic background or your race or occupation," Woodard said.

Symptoms extend far beyond normal sadness, and can include loss of interest in normal activities, agitation, insomnia or excessive sleeping.
If these symptoms become more severe and persist for at least two weeks, that's when Woodard says it becomes a major depressive episode. Depression can also exist in parallel with alcohol and drug abuse.
     
"Certainly people who are depressed can be self medicating using drugs or alcohol. Sometimes when people are using drugs or alcohol it can cause people to be depressed or anxious, so it can go either way," Woodard said.

He also says when addiction and depression become interwound, it can quickly spiral out of control.

"If you get so depressed or so down that you're never going to feel better, that there's no light at the end of the tunnel, that's when people are at risk," he said.

Sometimes because of the stigma that comes with depression and drug and alcohol addiction, Woodard says people fail to ask for help.
     
"What really important for people to know is it can get better and it will get better but you have to reach out for help, you have to let somebody know," Woodard said.

That wasn't necessarily the case with Robin Williams, but Woodard says if you do have a loved one suffering from depression or addiction, one should reach out to them. Embrace them, tell them they are loved, and guide them towards seeking professional help.