For years scientists and physicians believed that the rising rates of asthma across the globe were due to a cleaner world. That is, improved sanitary conditions meant that the immune system had less exposure to infectious agents and as a result defended itself against harmless substances like dust and ragweed. That reaction led to the breathing spasms and airway swelling associated with asthma.

But while this so-called “hygiene hypothesis” has held true for allergies, more recent research has chipped away at it as an explanation for asthma. Why? For starters, asthma rates have skyrocketed in less-than-clean U.S. urban areas. Also, asthma rates didn’t start spiking until the 1980s, well after sanitary conditions had improved in the world’s more affluent areas.

According to a story in Scientific American, experts now believe that asthma, which afflicts 300 million people around the world, is much more complicated than they originally thought. Only half the cases are related to allergies, and asthma may not even be a single disease. This has tremendous implications for its treatment and prevention, they say.

To read the complete Scientific American story, Go Here Now.