(CNN)Here is a look at autism.
Autism or autism spectrum disorder (ASD) incorporates a group of neurodevelopmental disorders causing impaired communication skills and social skills. ASD generally starts before three years of age and lasts a lifetime, but early intervention plays a role in treatment and progress.
ASD is about four times more common among boys than girls, according to the Centers for Disease Control and Prevention (CDC).
ASD can be found among all races, ethnicities and socioeconomic groups.
The prevalence of ASD in the United States is about one in 59 children, according to a 2018 CDC report.
Health care costs for children with autism are four to six times greater than medical costs for children without autism, according to research published in the Journal of Autism and Developmental Disorders.
There is no definitive medical test to diagnose autism. Instead, the disorder is diagnosed by observing a child's development.
According to the CDC, signs of autism may include deficits in social communication and interaction in a variety of contexts, difficulty engaging in back-and-forth conversation and an absence of interest in forming friendships with peers.
Vaccines and Autism
The debate over whether autism spectrum disorders are caused by vaccines started in 1998 when the medical journal The Lancet published a now-retracted study by researcher Andrew Wakefield linking the MMR vaccine to autism.
Most of Wakefield's co-authors withdrew their names from the study when they learned he had been compensated by a law firm intending to sue manufacturers of the vaccine in question. In 2010, Wakefield lost his medical license. In 2011, the Lancet retracted the study after an investigation found Wakefield altered or misrepresented information on the 12 children who were the basis for the conclusion of the study.
Other researchers have not been able to replicate Wakefield's findings. Several subsequent studies trying to reproduce the results have found no link between vaccines and autism, including several reviews by the Institute of Medicine.
Early 1900s - Autistic characteristics are studied as symptoms of schizophrenia.
1938 - Donald Gray Triplett of Mississippi is first examined by child psychiatrist Leo Kanner of Johns Hopkins Hospital and later becomes the first person diagnosed with autism symptoms.
1943 - Triplett is identified as "Donald T." in the paper "Autistic Disturbances of Affective Contact" by Kanner. The paper elaborates on the idea that autism is related to lack of parental warmth; this is later dubbed the "refrigerator mother" theory.
1944 - Hans Asperger, an Austrian physician, publishes a paper about autistic syndrome. The paper gains wider recognition when it is translated into English in the early 1990s.
1964 - Bernard Rimland, a research psychologist, publishes "Infantile Autism: The Syndrome and Its Implications for a Neural Theory of Behavior," which contradicts the "refrigerator mother" hypothesis. Kanner is the author of the foreword.
1965 - Rimland founds the National Society for Autistic Children (now the Autism Society of America). He later establishes the Autism Research Institute.
1980 - Autism is classified separately from schizophrenia in the third edition of the Diagnostic and Statistical Manual of Mental Disorders (DSM-III).
December 18, 2007 - The United Nations adopts a resolution declaring April 2 World Autism Awareness Day.
October 29, 2014 - The medical journal Nature reports that scientists have identified 60 genes with a greater than 90% chance of increasing a child's autism risk.
December 17, 2015 - Scientists at Harvard and MIT announce they have found, for the first time, a link between autistic behavior and reduced activity of a key neurotransmitter, a type of brain chemical that enables the transmission of signals across neurons, allowing the brain to communicate with other organs.
April 21, 2016 - The Simons Foundation announces that it is launching an autism research project called SPARK. The study, which involves scientists at 21 hospitals and university clinics, will focus on the possible connection between genetics and autism. Parents of children with autism are invited to sign up online and participate in the study. The group also works with Autism Speaks and the Autism Science Foundation to run the Autism BrainNet network, which is an autism brain bank that collects postmortem donations.
February 2017 - Researchers find that monitoring MRI brain scans of infants may help predict whether they will develop autism, according to a study published in the journal, Nature. The researchers found a possible link between brain enlargement during the first year of life and an autism diagnosis at age 2. This builds on previous similar research.
March 19, 2017 - CBS News' "60 Minutes" profiles "Sesame Street's" newest Muppet character, a girl named Julia who has autism.