The part of speech of “susceptible,” meaning “easily under a specified power or influence,” is adjective. It describes a person or thing that can be affected, harmed, persuaded, or changed by something else.
The word comes through French from Latin susceptibilis, meaning “capable of receiving,” from suscipere, “to take up” or “receive.” English uses it in phrases such as “susceptible to infection,” “susceptible to suggestion,” and “susceptible to heat.”
A common usage mistake is pairing it with the wrong preposition. The standard construction is usually “susceptible to”: “Young plants are susceptible to frost.” The related noun is “susceptibility,” as in “the patient’s susceptibility to disease.”
“Susceptible” is sometimes confused with “sensitive,” but the meanings are not identical. Someone who is sensitive may react strongly, while someone susceptible is especially vulnerable or open to being affected. In grammar, “susceptible” is an adjective because it modifies a noun or follows a linking verb, as in “a susceptible population” or “the population is susceptible.”