An estimated 121 million people are affected by some form of depression worldwide, with 80 percent of those affected not receiving any sort of treatment. That's an alarming number considering the damaging effects depression has on individuals.

Depression comes in many different forms with two being the most common: major depression (clinical) and dysthymia (chronic).

Major depression may reoccur and lasts nearly every day for at least a two-week period, while dysthymia is a less-severe, but longer-lasting form of depression. It doesn't necessarily last every day, but an individual cannot go more than two straight months without feeling depressed in a two-year period. According to the National Institute of Mental Health, major depression affects about 14.8 million Americans, while dysthymia affects 3.3 million.

Now while a mental health expert can tell when you're depressed, sometimes it's hard to notice the symptoms yourself. So it either goes undiagnosed or gets mistaken for just sadness. Left untreated, however, depression can result in serious problems, such as drug abuse, reckless decisions, deteriorating health or even suicide.

With that said, here are eight signs you could be depressed and not even know it:

1. Intense sadness that just won't fade

The difference between sadness and depression is that one is temporary while the other lingers. While people can typically get over whatever has caused them to feel sad, depression clamps down and does not let go that easily. It is a mental disorder rather than a simple emotion. If you are experiencing feelings of hopelessness, grief or unhappiness that are interfering with how you function on a daily basis, it's most likely because of depression.

2. Loss of interest in life or activities you once enjoyed

While there are people who experience feelings of intense sadness when they are depressed, that's not the case with everyone. There are many different symptoms of depression, so each individual may experience some but not all of them. In other words, even if you're not feeling sad, you could still be depressed. Feeling apathetic, numb or empty are also symptoms. They lead to no longer having any interest in doing things you once enjoyed.

3. Easily angered and quick to fight

Did you used to have more patience than you do now? Is it easier for people or incidents to make you snap? Do you now find yourself flying off the handle at every little thing? These are the results of increased irritability. As Katherine Krefft, a psychologist in Buzzards Bay, Massachusetts, states, "Sadness can be a cause of depression, but anger is a much more common cause, especially unrecognized anger that is not dealt with." If you find yourself involved in more arguments, especially ones that tend to escalate quickly, chances are you're struggling with unresolved anger, which has led to depression.

4. Neglecting everyday tasks or struggling to do them

When people are depressed, negative feelings are constantly weighing down on them, so it's common to feel exhausted and lethargic. This leads to them struggling to complete everyday tasks, such as cleaning their houses and maintaining their physical appearances. In some cases, they'll ignore doing them altogether because they don't feel motivated enough to put forth the effort.

5. Doing things excessively

Another sign that you've fallen into the dark pit of depression is when you find yourself doing things excessively that don't require much energy or social interaction, such as sleeping or eating. As for watching TV, a study published in The American Journal of Epidemiology found that people who watch more than three hours of it a day are more likely to be depressed. To a depressed individual, these actions are used as distractions. As for drinking alcohol excessively, it's sometimes used as a means of self-medication.

6. Overwhelming thoughts that produce a cynical outlook

When you're depressed, you feel as if you've fallen into a black hole of negativity and can't get out. Your thoughts become scattered and harder to control. As a result, you end up finding it harder to concentrate or remember things. Maybe you've been finding yourself lying awake at night with restless, troublesome thoughts, which has contributed to a case of insomnia — another common symptom associated with depression. Just as doing other activities excessively is a red flag, so is having excessive negative thoughts.

7. Bitterness toward others who appear joyful

Growing up, I noticed my mother had this "bad habit" of talking bad about others or cursing them bitterly if they appeared to be happy with their lives. It wasn't until later when she was diagnosed that I realized she had severe depression. Her bitterness and jealousy were a result of seeing others happy and feeling like she could never achieve that. Again, depression can cause you to feel hopeless.

8. Would rather be alone than with people

Maybe you used to socialize a lot more than you do now. Ever wonder why you've suddenly done a 180 and now prefer not to be around people? Or maybe you've never been much of a sociable person, but you find yourself feeling more reserved than usual. In either case, some people with this mental condition have no desire to be around others. They shut down and shut others out, which in fact only makes it worse.

So if you notice some of these signs within yourself or others close to you, it's a possibility depression is involved. If you're the one exhibiting these behaviors but still unsure whether you should get yourself checked out by a mental health professional, you can take this free depression test that provides scores for each of the six depression disorders to give you an idea of what form of depression you may be experiencing, if any at all.

Here are eight suggestions that may help pull you out of depression if you are experiencing it:

• Seek a professional or someone close you can confide in
• Find a creative outlet such as a hobby you've enjoyed doing
• Stay connected to the outside world, especially your family and friends
• Exercise regularly to let off steam or stress
• Avoid alcohol or any other drug substance not prescribed to you
• Socialize with people who provide a positive influence in your life
• Help out others or volunteer somewhere to get the focus off of you
• Join a support group

Now while I have personally dealt with depression on and off throughout my life, I am by no means a mental health expert. So if you think you might be depressed, it's best to get a professional evaluation and find out for yourself, so you can receive the treatment you need.