(GRE Success! ) I don't understand. Can you figure it out?

Hello friends, 
I find this sentence in NYTIMES "Health" section. 

"Among the many mysteries of human biology is why complex diseases like diabetes, high blood pressure and psychiatric disorders are so difficult to predict and, often, to treat". 

I don't understand structure of this sentence. Can you figure it out? 

For detailed article, you can follow this link: http://www.nytimes.com/2012/09/06/science/far-from-junk-dna-dark-matter-proves-crucial-to-health.html?_r=1&pagewanted=all 


Have a great time ahead. 

See you soon. 

With warm regards, 
Rajesh Chaudhary