The number that this article from PsychCentral reports is quite scary. And the article probably could have been written in a more positive way: it talks nothing about what the root causes are and how to prevent it. It's only talking about the fact that it needs to be addressed by health care to take care of those kids and analyzed as for the impact that this will have in the economy when those children arrive at the workforce.
So I went out looking for articles talking about the possible sources for autism spectrum disorder. I'm not a medical doctor, so I'm certainly not the best person to rate these articles, but I found one that was quite complete-looking:
There are lots of references out there, and many from not-so-impartial sources, like nutrition websites (that sell food supplements) that claim that it's because people don't do a complete detox before having kids; or genetics labs that keep pounding on the evidence of a genetic link to chance of developing autism (in many ways, everything is genetic, but genetics can't explain a 10-fold increase in number of autistic kids in a period of less than 10 years!); or some immunology magazines that keep repeating that there is no proof that there is a connection between autism and vaccination (there was a huge controversy about that some years ago because of a research that suggested that there was a connection).
In other words, people still don't know. It's probably somehow related to the immune system, but if it's a cause or just a correlation is hard to tell. Again, I'm still not sure how something like this could cause such a huge spike in the number of cases. But who am I to know those things?