Christ came to the world to “bear the infirmities” of men. One of the signs of his divine messiahship was to heal the sick. The power of healing remains in the Church since Christ himself remains in the Church through the Holy Spirit.

The sacrament of the unction of the sick is the Church’s specific prayer for healing. If the faith of the believers is strong enough, and if it is the will of God, there is every reason to believe that the Lord can heal those who are diseased.

https://oca.org/orthodoxy/the-orthodox-faith/worship/the-sacraments/holy-unction