This is a very complex issue and goes back to the old 'nature versus nurture" fight.
Yes, autism is a nuerological disease but saying that it is "only" a genetic component is "wreckless". As with many other diseases, enviromental aspects can have an influence in the disease.
There has yet been any conclusive scientific evidence that immunizations are or can be the sole cause for autism. These conclusions are theory only and in no way have been supported by peer reviewed medical research.
There is, however, research, stating that there is no relationship between thimerosal and autism.
It could be that those who are genetically predisposed but not showing any signs (carriers) could be influenced by the mercury in vaccines, but this is not been shown true or false as of yet. If it is true, it would be a small percentage of the population and then they would have to identify the genetic component before those individuals could be identified at risk. Most causes of autism are idiopathic, meaning there is no known cause.
I would, as of yet, be "blaming" vaccines. It is a complex issue, and with no conclusive evidence to pin point vaccines as the sole or primary cause.
I do agree that the typical western lifestyle does not help in any of the health issues. The matter is much more than "fast food" though.
It is the modern convienences, the lack of excercise, the high fat highly processed diet, and the lack of careing about overall health that is the main concern for any disease. People think that in the abscence of disease, there is health, and this is not true.
We need to be mindful about health, but not to the point of making ourselves and our children more susceptible to disease. Afterall, too much hand washing and anti-bactirial products can actual kill the good bacteria we need to survive.