Probably by backformation from susceptible.
suscept (plural suscepts)
- (biology) A species which is capable of being infected or parasitized by another species.
- As is self-evident given its name, humanity is a suscept for HIV.
Suscept is used only to refer to a species as a whole, but not to individual organisms that have been infected or parasitised. The term host is used to describe either the species as a whole or specific organism that has been infected or parasitized.