There are two main characteristics commonly found in the poetry of the Metaphysical Poets. These characteristics are both wit and the use of metaphors/similes (being either simple ones or unusual ones). As for the poem itself, it was normally written using simple verse.
Some of the most well-known Metaphysical Poets are John Donne, George Herbert, and Andrew Marvell.
Both poems are titled to lead the reader to believe that the poem is about either a flea or a fever. Instead, given the importance of the metaphor in Donne's poetry, each is not about a flea or a fever. Instead, the poem about the flea can be interpreted to be about how love will drain a person of life. The poem about a fever can be interpreted to be about how one needs to be intense with their life, much like the heat of a fever.