Clinical depression is extremely common around the world, with an estimated 350 million cases reported by the World Health Organization.

People of all ages suffer from depression, which is a leading cause of disability. The mental disorder, says the WHO, is linked to about 1 million deaths every year from suicide or some other cause, such as substance abuse.

There are effective treatments for depression. But some, like lithium, carry significant and even dangerous side effects.

So researchers are now looking at a number of unconventional approaches to treating depression.

In Britain, scientists think some of the vast number of microbes that live in the gut may play a role in the development of mood disorders. They report that so-called probiotics, or helpful bacteria, relieve anxiety in mice and lower levels of the stress hormone cortisol.  

In humans, probiotic supplements have been shown to boost "good" bacteria in preliminary research, reducing bacteria that trigger inflammation. Scientists think the beneficial microbes may improve the way people process emotions, resulting in less anxiety. They plan further studies.

Meanwhile, a research consortium of a half dozen universities in the U.K. is focused on the role inflammation may play in clinical depression, exploring a newly discovered link between immune disorders, such as diabetes and rheumatoid arthritis, and mental illness.  

Investigators say up to 40 percent of patients who receive interferon to treat a liver infection called hepatitis become depressed even though they were fine before treatment. Interferon triggers inflammation.

Researchers stress that the link between inflammation and clinical depression is not yet understood.

It's hoped that new drugs that reduce the inflammatory response can help ease depression or that existing anti-inflammatory drugs can be repurposed to help those who suffer from the mental disorder.