Selective Mutism and My Personal Experiences With It
Selective Mutism is a condition in which a child who can speak and who often speaks at home, will not speak in public settings, most commonly in school or social settings. This anxiety disorder is most commonly found in children under 5. Unlike Mutism, where a child never speaks, the child does speaks at home with close family members. Selective Mutism becomes mentally and emotionally distressing to a child if it is not treated with urgency. Many parents and teachers think the child does not speak simply because he/she does not want to speak. In fact, the child is truly unable to speak in these certain settings and situations. The causes of Selective Mutism are not known, but children with it do seem to exhibit symptoms of extreme social phobia. 
I used to have Selective Mutism, along with my twin sister, and my parents found out as soon as we started talking that we would freeze up with a paralyzing fear in any type of social setting. I am lucky they recognized this when they did, and that they realized that it was more than us not wanting to speak, but it was that we had a fear of speaking to others.
I remember feeling this paralyzing fear when I had to talk to "strangers" (like my school teachers or peers in my classroom). My grandma told me that I described this fear as "having the words stuck in my throat". I remember wanting to talk, but physically not being able to do so. I literally felt as if the words were stuck in my throat. I was described as extremely shy.
My parents worked with my teachers to help me overcome my anxiety. They invited my teachers to my home, made videos of my twin sister and I talking and sent them to my teachers (mostly so my teachers didn't think we were unwilling to speak or "dumb"), and took both my twin sister and me to a psychiatrist. We had our daily dose of medicine in our orange juice every morning, and soon after, my parents started to see improvement in us. We started whispering to our teachers and classmates! What a step ahead! I remember whispering my lunch order in my 1st grade teacher's ear and she got so excited that she wrote a note home to my parents.
I have improved quite exponentially in this condition because of my wonderful parents and teachers. But there is no doubt that I feel the effects of this disorder today. But without them, who knows where I would be.
It is so important to know about this disorder, especially for anyone involved in careers or activities that involve children. The main reason is this: if we force a child with Selective Mutism outside of their shell, they will only fall deeper into their shell. It has a negative effect. They will take 2 steps forward, and with one negative remark or statement, take 3 steps backwards. People who have no knowledge on this anxiety and think it's a "phase" or "just shyness" seem to be the ones that force a child to speak or look down on the child for not speaking. I've had my fair share of those encounters, and still do at times. It only made me want to run and hide in my comfortable shell of not talking. I felt defeat and failure in something I tried so hard to overcome. No child should feel that way.
That is why awareness is important, not only for this type of anxiety, but for any type of health issue with any type of person.