It might sound strange that a non-infectious disorder is reaching epidemic proportions around the world, but over the last few decades, there's been an explosion in the number of young people who have developed nearsightedness, and scientists are trying to explain why. In the US, around 40-50 percent of young people are being diagnosed as nearsighted, but that's nothing compared to kids in Asian countries such as Singapore, China, Japan, and Korea, where nearsightedness among high school students is around 80-90 percent. So what's going on?

Researchers have predicted that by 2050, half the world’s population (around 5 billion people), will likely be nearsighted, or myopic. One-fifth of those will be diagnosed as being in the high myopic category, which comes with an increased risk of blindness and cataracts.

Already, we're seeing a significant increase in the proportion of young people developing the disorder, with Elie Dolgin reporting over at Nature that just 60 years ago, 10-20 percent of the Chinese population were nearsighted, and now up to 90 percent of teenagers and young adults are, while in Seoul, an incredible 96.5 percent of 19-year-old men are being diagnosed as myopic.

In the US and Europe, the proportion of young people with myopia has doubled over the past 50 years. "We are going down the path of having a myopia epidemic," Padmaja Sankaridurg from the Brien Holden Vision Institute in Sydney, Australia told Dolgin.

As the episode of SciShow above explains, nearsightedness is caused by an elongation of the eyeball, which means that when light hits the eye, the lens focusses it in front of the retina, instead of right on its surface. This means anything that's further away than your outstretched arm will appear slightly blurry.

The good news is that, despite what scientists used to think, nearsightedness isn't wholly caused by our genetics, which means we have some control over whether or not we develop it. And the prevalence of technology such as smartphones and tablets - especially in young people - is likely not the main culprit either, because the increase in nearsightedness happened long before these things became mainstream.

In fact, extensive studies have failed to prove a direct link between nearsightedness and 'near work' - peering too closely to written words - so there must be something other than us glued to our computers all day and our genetics at play here. As the video explains, scientists have found a link between a lot of education and a higher risk of nearsightedness, but they say it's not all that reading and computer time that's likely the cause - it's how much time kids spend outside.

Watch SciShow above to find out why this could be the case, and why plain old sunlight could lower our overall risk.