For example, learning and reasoning can be especially difficult for many kids with autism, but others may learn quickly. In the United States, over 500,000 people have autism or one of several other similar disabilities. Kids who have autism usually start having symptoms between the ages of 18 and 36 months. More boys have autism than girls. Autism isn't contagious, which means you can't catch it from someone who has it.
Nobody knows for sure what causes autism.
Some scientists think that some kids might be more likely to get autism because it or similar disorders run in their families. Knowing autism's exact causes is hard because the human brain is very complicated. Each neuron may have hundreds or thousands of connections to other nerve cells in the brain and body. Scientists are still trying to understand how and why this happens. Figuring out if a kid has autism can be difficult.