Landmarks around the globe are being illuminated in blue lights on Thursday to mark World Autism Awareness Day 2015. The sixth annual "Light It Up Blue" campaign is a striking visual reminder of the challenges autism poses to millions of people and their families, and of how much more work needs to be done to understand the developmental disorder.

One in 68 children in the U.S. is diagnosed with an autism spectrum disorder, according to the latest figures from the Centers for Disease Control and Prevention. The statistics, released last year, were based on data collected in 2010 from 8-year-olds in 11 U.S. cities. They reflected a nearly 30 percent increase in rates of autism spectrum disorders over the course of only two years.

The increase is partly due to greater awareness among parents and doctors, but experts say the condition also appears to be occurring more frequently for reasons that are not well understood.

A child creates art at an event for World Autism Awareness Day in Himeji, Japan, on April 2, 2015. Buddhika Weerasinghe, Getty Images

Autism spectrum disorders are a group of developmental issues that affect a person's social, communication and behavioral functioning. The diagnosis includes milder forms of the disorder such as Asperger's syndrome.

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While researchers continue their efforts to identify how and why autism develops, some are seeing success with early intervention programs. One study published last fall found that if treatment begins as early as the first six months of life, it can vastly improve a child's outcome and possibly even eliminate symptoms.

In 2007, the United Nations designated April 2 as World Autism Awareness Day to help spotlight this growing global health crisis.

"Together, we must spread a message of compassion, understanding and hope for the global autism community by supporting comprehensive strategies that address their needs throughout a lifetime," Suzanne and Bob Wright, co-founders of the advocacy group Autism Speaks, said in a statement. They called for worldwide collaboration to speed progress for people with autism and their families.