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The prevalence of autism is about 1–2 per 1,000 people worldwide (the number of boys affected exceeds the number of girls 3-5 fold); however, the Centers for Disease Control and Prevention report approximately 9 per 1,000 children in the United States are diagnosed with autism spectrum disorders (ASD). The number of people diagnosed with autism has increased dramatically since the 1980s, partly due to changes in diagnostic practice. The question of whether actual prevalence has increased is unresolved.

Autism has a strong genetic basis, although the genetics of autism are complex and it is unclear whether ASD are explained more by rare mutations, or by rare combinations of common genetic variants. In rare cases, autism is strongly associated with agents that cause birth defects. Controversies surround environmental causes, such as heavy metals, pesticides or childhood vaccines.

Autism is a highly variable neurodevelopmental disorder that first appears during infancy or childhood, and generally follows a steady course without remission. Overt symptoms gradually develop after the age of six months, become established by age of two or three years, and tend to continue through adulthood, although often in more muted form. Autism is distinguished not by a single symptom, but by a characteristic triad of symptoms: impairments in social interaction; impairments in communication; and restricted interests and repetitive behavior.

About a third to a half of individuals with autism do not develop enough natural speech to meet their daily communication needs. Differences in communication may be present from the first year of life, and may include delayed onset of babbling, unusual gestures, diminished responsiveness, and vocal patterns that are not synchronized with the caregiver. Autistic children may have difficulty with imaginative play and with developing symbols into language.

The hallmark feature of ASD is impaired social interaction. As early as infancy, a baby with ASD may be unresponsive to people or focus intently on one item to the exclusion of others for long periods of time. A child with ASD may appear to develop normally and then withdraw and become indifferent to social engagement.

Children with an ASD may fail to respond to their names and often avoid eye contact with other people. They have difficulty interpreting what others are thinking or feeling because they cannot understand social cues, such as tone of voice or facial expressions, and don't watch other people's faces for clues about appropriate behavior. They lack empathy.

Autistic individuals display many forms of repetitive or restricted behavior, which the Repetitive Behavior Scale-Revised (RBS-R) categorizes as follows.

• Stereotypy is repetitive movement, such as hand flapping, making sounds, head rolling, or body rocking.
• Compulsive behavior is intended and appears to follow rules, such as arranging objects in stacks or lines.
• Sameness is resistance to change; for example, insisting that the furniture not be moved or refusing to be interrupted.
• Ritualistic behavior involves an unvarying pattern of daily activities, such as an unchanging menu or a dressing ritual. This is closely associated with sameness and an independent validation has suggested combining the two factors.
• Restricted behavior is limited in focus, interest, or activity, such as preoccupation with a single television program, toy, or game.
• Self-injury includes movements that injure or can injure the person, such as eye poking, skin picking, hand biting, and head banging. A 2007 study reported that self-injury at some point affected about 30% of children with ASD.

No single repetitive or self-injurious behavior seems to be specific to autism, but only autism appears to have an elevated pattern of occurrence and severity of these behaviors.
Several other conditions are common in children with autism. They include:

• Genetic disorders. About 10–15% of autism cases have an identifiable Mendelian (single-gene) condition, chromosome abnormality, or other genetic syndrome, and ASD is associated with several genetic disorders.
• Mental retardation. The fraction of autistic individuals who also meet criteria for mental retardation has been reported as anywhere from 25% to 70%, a wide variation illustrating the difficulty of assessing autistic intelligence. For ASD other than autism, the association with mental retardation is much weaker.
• Anxiety disorders are common among children with ASD; there are no firm data, but studies have reported prevalences ranging from 11% to 84%. Many anxiety disorders have symptoms that are better explained by ASD itself, or are hard to distinguish from ASD's symptoms.
• Epilepsy, with variations in risk of epilepsy due to age, cognitive level, and type of language disorder.
• Several metabolic defects, such as phenylketonuria, are associated with autistic symptoms.
• Minor physical anomalies are significantly increased in the autistic population.
• Preempted diagnoses. Although the DSM-IV rules out concurrent diagnosis of many other conditions along with autism, the full criteria for ADHD, Tourette syndrome, and other conditions are often present and these comorbid diagnoses are increasingly accepted.
• Sleep problems affect about two-thirds of individuals with ASD at some point in childhood. These most commonly include symptoms of insomnia such as difficulty in falling asleep, frequent nocturnal awakenings, and early morning awakenings. Sleep problems are associated with difficult behaviors and family stress, and are often a focus of clinical attention over and above the primary ASD diagnosis.

It is common knowledge that autistic children suffer from:

• Insufficient perfusion (reduced blood flow to the brain resulting in the lack of oxygen and malfunctioning).
• Immune disorders.

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