Sylvia has the flu. the ongoing achiness in her joints reflects the action of pain fibers.

The ongoing achiness in Sylvia's joints reflects the action of alpha delta pain fibers.
Alpha delta fibers are nociceptrors whose function is primary transmit acute pain to the central nervous system. During a flu, the white cells of the immunological system produce cytokines to fight the infection, but a side effect of the cytokines is joint inflammation which causes alpha delta fibers to transmit the information of acute pain to Sylvia's brain.