Is autism contagious?

No, but autism diagnoses are, in a sense.

The Columbia University team looked at data on over 300,000 children born between 1997 and 2003 throughout California. The team found that (C)hildren who live within 250 meters of a child with autism have a 42 percent higher chance of being diagnosed with (autism) in the following year compared with children who do not live near a child with autism. Children who live between 250 meters and 500 meters from a child with autism were 22 percent more likely to be diagnosed. The chances of being diagnosed decrease significantly the farther children live from another child with autism.

The study used several tests to determine whether these results could be explained by a social influence effect, or if environmental toxicants or a virus are to blame. For example, the researchers looked at children who live close to each other, but on opposite sides of school district boundaries. These children are likely exposed to the same environmental conditions, but their parents likely belong to different social networks.

The research shows that the increased chance of diagnosis only exists when parents reside in the same school district. Children who live equally close to a child with autism—but in another school district—were no more likely to be diagnosed with the disorder than children who do not have a neighbor with autism. The results are a strong indication that the proximity effect is a social phenomenon and not the result of environment, Dr. Bearman says. 

The reason, they say, is that most people don't really know about autism, so they don't know to get a screening, unless they live close to someone with an autistic kid, who can tell them what to do and guide them through the system.

This makes intuitive sense. People I hang out with are immersed in kid stuff, and most of them can tell you offhand the symptoms that would lead to a referral for all sorts of conditions, but most parents aren't and can't.