Good question. Most scientists believe that there is both a genetic susceptibility AND an environmental trigger. I am not a scientist, but my personal opinion is that autism is caused by some combination of genetics, health status when vaccinated, environmental factors, and vaccines.
Consider these possible risk factors:
There are certain genetic disorders that interfere with the body's ability to remove heavy metals, such as Wilson's Disease which causes the patients' bodies to store dangerous levels of copper. (Interestingly enough, this study found that patients with Wilson's disease may also be more sensitive to aluminum:
"Interaction of trace metal metabolism was studied in a patient with Wilson's dease. Atomic absorption analysis showed markedly increased urinary excretion of copper and aluminum and an increased aluminum content was found in the biopsied liver by neutron activation analysis. These findings suggest a complicated pathogenetic mechanism involving other metals besides copper in the Wilson's disease." Here)
Also, one study found that, "Aluminum absorption is enhanced in the presence of elevated circulating parathyroid hormone. In addition, elevated PTH leads to the preferential deposition of aluminum in brain and bone. Consequently, PTH is likely to be involved in the pathogenesis of toxicities in those organs. PTH excess also seems to lead to the deposition of aluminum in the parathyroid gland. The in vitro demonstration that aluminum inhibits parathyroid hormone release is consistent with the findings of a euparathyroid state in dialysis patients with aluminum related vitamin D-resistant osteomalacia. Nevertheless, it seems likely that hyperparathyroidism is at least initially involved in the pathogenesis of aluminum neurotoxicity and osteomalacia; the increases in tissue aluminum stores are followed by suppression of parathyroid hormone release, which is required for the evolution of osteomalacia. Impaired renal function is not a prerequisite for increased tissue aluminum burdens, nor for aluminum-related organ toxicity. Consequently, it is likely that these diseases will be observed in populations other than those with chronic renal disease." Here
Or if there is no genetic link, prior toxic doses of thimerosol are believed to reduce the body's ability to produce glutathione. Children with autism have been found to have low levels of glutathione. Glutathione is required by the body to remove toxins and fight viruses. Low glutathione levels would make the aluminum-containing vaccines and live-virus vaccines more dangerous for the affected children.
Health Status at the time of vaccination:
The CDC now says that "Children who are severly ill should wait until they recover before being vaccinated". We know that now, but for many years illness wasn't considered a serious risk for vaccines.
Severe dehydration negatively impairs kidney function, and reduced kidney function is an acknowledged risk factor for aluminum toxicity. (See the posts about aluminum on my blog.)
Also, other studies have found that certain vitamin deficiencies impair the body's ability to remove toxins. Children with gastrointestinal problems typically suffer from vitamin deficiencies, so GI problems could make vaccines more risky for these individuals. GI problems are commonly seen in children with autism.
A recent study found that the risk of autism increased in direct correlation to how close the patient lived to a power plant due to mercurial pollution.
A higher level of enviromental mercury exposure combined with the toxins found in vaccines would create a higher risk of reaching toxic levels.
And this study reports finding a "significant association between the concentration of aluminum in drinking water and the incidence of dementia and Alzheimer's disease".Here
FYI: Some vaccines contain(ed) both mercury and aluminum. Mercury thermometers are not allowed on some airliners because mercury damages the protective oxide surface of aluminum.
The contents of each vaccine vary by brand. For example, the amount of alumininum in the 3 DTaP vaccine brands varies from 170 mcg to 625 mcg.
There is also the risk of "hot lots" (bad batches of vaccine). These are recalled, but usually not until after some children have been injured.
The autism percentage rate has increased every time a new aluminum-containing or live-virus childhood vaccine has been added to the schedule. See the Autism Rates and Vaccine Histories chart. Here
Also, see this safety data sheet for thimerosal, (here) which lists aluminum under the reactivity and stability section as a substance to be avoided.
And for me, the most compelling evidence against the use of aluminum in vaccines is the neurological delays in preemies given aluminum-containing medical treatments, dementia in kidney dialysis patients (also caused by aluminum), and the link between Alzheimer's disease and aluminum. It's still a controversial debate as to whether aluminum causes Alzheimer's or if Alzheimer's disease causes the patients' brains to accumulate aluminum. HOWEVER, in both Alzheimer's disease and regressive autism, the patients loose previously aquired skills.
Any combination of these risk factors could produce vast differences to childrens' responses to vaccines.
A best-case scenario would include:
No genetic susceptibility, patient in perfect health when vaccinated, lives in an area with little to no pollution, and only receives the vaccine brands with the least toxic ingredients. This patient would probably be fine after receiving the routine vaccines.
By contrast, here is a worst-case scenario example:
A child with a genetic susceptibility or glutathione deficiency, severly ill at the time of vaccination, lives in an area with high pollution levels, and receives all the vaccine brands that contain the highest levels of toxic ingredients. This child could conceivably be at high risk of an adverse vaccine reaction, possibly even including permanent brain damage.
(However, I am not a doctor. Always consult your doctor before making any medical decision. I am merely stating that vaccines need to be researched further.
I am also not anti-vax. I do think that some vaccines, given at a much slower rate than the current schedule, are valuable. On the other hand, I think some vaccines are not worth the risk. You can read The Vaccine Book by Dr. Sears if you want advice on which vaccines are right for your child.)
If vaccines are indeed THE trigger, then the variance in all these risk factors would help explain the wide spectrum seen in Austim cases.