In context|biology|lang=en terms the difference between host and facultative
is that host
is (biology) a cell or organism which harbors another organism or biological entity, usually a parasite while facultative
is (biology) able to perform a particular life function, or to live generally, in more than one way.
As a noun host
is one which receives or entertains a guest, socially, commercially, or officially or host
can be a multitude of people arrayed as an army; used also in religious senses, as: heavenly host (of angels)
can be (catholicism) the consecrated bread or wafer of the eucharist.
As a verb host
is to perform the role of a host.
As an adjective facultative is
of or relating to faculty, especially to mental faculty.