A corporation may be organized for the purpose of establishing, maintaining, and operating a nonprofit hospital service plan whereby hospital care may be provided by a hospital maintained by a corporation organized for hospital purposes to such of the public who become subscribers to such plan under a contract which entitles each subscriber to certain hospital care. As used in this chapter, the term "hospital service corporation" includes any corporation organized under the provisions of this chapter and also any unincorporated association furnishing hospital, medical, surgical, or nursing services, or any combination of the foregoing, to subscribers or members, except as provided in section 4519 of this title.