<The Lord and the beloved disciple.
Also, Author of a Gospel. Among St. John’s enormous contributions to the Kingdom of God (just imagine: to him was entrusted, by our Lord Jesus Christ from the Cross, the care of the Most Holy Theotokos!) he intercedes for us in Christ’s Presence. In terms of specific disabilities, he is counted in the West as an intercessor for burn victims.
A short summary would not do justice to St. John. There are plenty of online Orthodox Christian narratives of his life. Choose one, and read it through. Here are four options:
The Roman Catholic site which lists the many things St. John is noted as an intercessor for: http://saints.sqpn.com/saint-john-the-apostle/