My 8 year old daughter has Autism and associated disorders and gastric problems, but that doesn't mean she's "sick." Referring to Autism and other neurological disorders as an illness is tricky territory, just so you know, so please don't refer to them as being sick. And do not put disorders in quotation marks. It insinuates that you don't believe they're actually legitimate disorders. Please think about how you phrase things before asking questions on sensitive topics in the future.
More people are being diagnosed with Autism Spectrum Disorders because there's more research being done.
There's more literature out there than there was even ten years ago.
There are more professionals specializing in ASD's than there were ten, fifteen, twenty years ago.
Many of these professionals (including special education teachers, school psychologists, sociologists, social workers, etc) are integrated in the public schools and are recognizing students in need much sooner than before.
Parents have better access to information than ten or fifteen years ago and are better equipped to recognize signs of Autism and bring questions to their doctors. Access to this information also means that parents are better able to advocate for their children when they suspect a neurological disorder such as ASD.
There are some misdiagnoses, but those don't make up the majority or even a large minority of the Autism community. Autism is a spectrum disorder, so someone can be very, very mild with their symptoms so as to be virtually unnoticeable or they can be very severe with their symptoms. Most people with ASD are in the Bell Curve. So someone can have a diagnosis of ASD and not "seem" to have it because their coping skills are excellent or their symptoms are mild.
As for actual illnesses with germs and immune system problems and allergies, that could probably be accounted for with a few things.
For a long time, even now, the majority of babies were/are fed with formula over breastmilk which has natural antibodies in it.
Our diets in general as Americans are not very good. Americans tend to buy cheap to eat cheap, and shows up as obesity and health problems. There are some genetic health problems that obviously would crop up anyway, but they have a higher likelihood when our diets and stress levels are so poor.
Edit: I just have to add that my mother and her mother had very natural pregnancies and medication-free deliveries. I was born quite prematurely. My daughter with ASD was born somewhat prematurely, and I went into premature labor twice before she was born. I had medicine-free pregnancies with all of my children. My ASD daughter showed signs of Autism at BIRTH with sensory processing disorder (look it up if you don't know what it is, and for her it's always been very severe). Before she ever had a single vaccination or feeding of anything. I ate amazingly well for all pregnancies. Genetics played a huge role in her luck of the draw as do many, many neurological disorders.
Something doesn't always have to be blamed on something else. In most cases, it's genetic. There's an awful lot of evidence that supports that. Environmental factors may exacerbate it, as does diet, and allergies to foods and intolerances plus sensitivities to foods and dyes and HFCS can mimic signs and symptoms of ADHD and Autism as well as make them worse if ADHD and Autism already exist.
So please don't assume that ASD is entirely environment or about diets or even about vaccinations. The business with "Dr." Andrew Wakefield claiming a so-called link between vaccinations and Autism was thoroughly debunked and scientifically proven as false as well as being proven that his entire "study" and article in the Lancet as being a hoax. This has all occurred within the past year. There is no link in this particular issue. That's not to say that vaccines may not be linked to other disorders, but they're not linked to Autism. The Lancet even retracted the Wakefield article this past year and apologized for having printed it in the first place all those years ago. The damage caused by that article is unmeasurable and worldwide.