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Thank you for your question and welcome to Just Answer. The proper term for your condition is "internal resorption". This occurs sometimes after the tooth has experienced some kind of trauma (physical blow, decay, bacterial infiltration from a crack) but many times has an unknown etiology. You did nothing wrong and were not negligent in your oral hygiene to get this condition. It will not spread to other teeth and the treatment consists of doing a root canal to rid the tooth of the aggressive pulpal tissue which is dissolving your tooth from the inside out. Treatment is normally very successful. Long term retention of your tooth depends on how much of your tooth remains after the nerve tissue remains. If all that remains is just a shell of a tooth, these need to be extracted as there is nothing left strong enough to support a crown or filling. OBviously your dentist thinks the tooth is restorable or he wouldn't have referred you to the specialist to have it treated.
I hope this answers your question. If you would like to discuss this further or have any additional questions, please reply to my answer and I will get right back to you.
Sincerely, XXXXX XXXXX DDS