Selective mutism is NOT Autism

Most people (especially Africans) usually mistake one disorder for the other. Although they both involve social anxiety, autism is a lifelong neurological development disorder, while selective mutism is a severe anxiety disorder, or situational communication disorder. They are both very different. A person with selective mutism doesn't speak in certain situations or to certain persons, although they are fully capable of speech and can understand languages. They may come off as being rude or shy, but in actual sense, they want to speak up but find it difficult to.
The cause of selective mutism is unknown yet diverse, in the sense that anything could be the cause ( trauma is not a likely cause), and at the same time, nothing could be the cause (it could happen to anyone). A child with this disorder might be silent in the classroom but speak freely at home or the reverse is the case. Hence, there is a sort of hierarchy in this, a child with selective mutism might be active in social activities but doesn't speak to anyone, or doesn't speak to adults but peers, or answers quietly in monosyllables to adults but doesn't speak to peers. Teachers often mistake this disorder for shyness or stage fright in the case of the child failing a public speaking assignment.
Persons with this disorder usually exhibit the following symptoms:

• Blank facial expressions and reluctance to smile

• Isolation and difficulty expressing feelings

• Sensitivity to noise or crowd

• A strong sense of right and wrong

• Above Average intelligence and high level of inquisitiveness

• Creativity and love for art or music

• Empathy (ironically).
  The main treatment for this disorder is "Behavior Therapy". It involves gradually exposing the child or person to increasingly "difficult speaking tasks" in the context of a supportive relationship, like from signs, to whispering, to one word replies, like progressively climbing up a ladder. However, the person with this disorder should be taken to a psychologist for other therapeutical treatments.

  Autism on the other hand, is a highly variable neurodevelopmental disorder that first appears during infancy or childhood, and generally follows a steady course without remission. It is characterized by:
  

-Imparied social interactions (mutism)
-Verbal and nonverbal communication
-Restricted and repetitive behavior (hand flapping, body rocking, placing objects in a specific order, hand washing, resistance to change and ritualistic behavior, and restricted interests, like a particular game, toy or TV program). The diagnostic criteria requires that symptoms become apparent in early childhood, typically before age three.
Autism is due to a combination of genetic and environmental factors. Some cases are strongly associated with certain infections during pregnancy, including rubella and use of alcohol or cocaine. Autism affects information processing in the brain by altering how nerve cells and their synapses connect and organize. Noted autism spokesperson, Mary Temple Grandin described her inability to understand the social communication of neurotypicals or people with normal neural development, as leaving her feeling "like an anthropologist on Mars". Autistic infants show less attention to social stimuli, smile and look at others less often, and respond less to their own names. Autistic toddlers differ more strikingly from social norms, for example, less eye contact and turn taking is a problem, and also the inability to use simple movements to express themselves, such as pointing at things. Several other conditions are common in children with autism like:

• genetic disorders
• intellectual disability
• anxiety disorders
• epilepsy
• several metabolic defects
• minor physical anomalies
• and sleep problems.
  This disorder is one of the five pervasive developmental disorders (PDD), of the five PDD forms, Asperges syndrome is closest to autism in signs and likely causes.
  Medication can be prescribed such as psychoactive drugs or multiconvulsants, stimulants and antipsychotics such as risperidone and aripiprazole. However, their side effects must be weighed against their potential benefits, and people with autism may respond atypically. There is no known cure for autism, although many alternative therapies and interventions are available, few are supported by scientific studies.
  The internet has helped autistic individuals bypass nonverbal cues and emotional sharing that they find so hard to deal with, and has given then a way to form online communities and work remotely.
  Sociological and cultural aspects of autism have developed in the sense that some in the community seek a cure, while others believe that autism is simply another way of being.