Dictionary.net

• One normally parasitic, but capable of living alone.

1898 - American pocket medical dictionary
By Willam Alexander Newman Dorland

• A p. that is not confined to any particular animal organism.

• A p. which can only satisfy the conditions of its life in the animal organism, but which nevertheless leaves it at a certain period of its existence.

1916 - Appleton's medical dictionary
By Smith Ely Jelliffe