LONDON (Reuters) - Europeans are plagued by mental and neurological illnesses, with almost 165 million people or 38 percent of the population suffering each year from a brain disorder such as depression, anxiety, insomnia or dementia, according to a large new study. With only about a third of cases receiving the therapy or medication needed, mental illnesses cause a huge economic and social burden - measured in the hundreds of billions of euros - as sufferers become too unwell to work and personal relationships break down. "Mental disorders have become Europe's largest health challenge of the 21st century," the study's authors said.