A noun appearing as N. fliw “sickness” in The Etymologies of the 1930s derived from ON. thlīwe < ᴹ✶slīwē under the root ᴹ√SLIW “sickly” (Ety/SLIW). The ancient initial sl became thl which was the usual sound change in (Old) Noldorin, and then this thl became fl, a less common Noldorin sound change. There was also a deleted variant of this root: ᴹ√LIW, where Tolkien had lhîw “disease” with the usual unvoicing of initial l in Noldorin (EtyAC/LIW).
Neo-Sindarin: In Sindarin of the 1950s and 60s, initial sl became lh instead of thl, so most Neo-Sindarin writers adapt this word as ᴺS. lhîw, as suggested by Hiswelókë’s Sindarin Dictionary (HSD). Based on the deleted variant of the root, I would use the word lhîw for both “sickness” and “disease”, both within a body and independent of it.
A noun for “sickness” in The Etymologies of the 1930s under the root ᴹ√KWAM (Ety/KWAM). For purposes of Neo-Sindarin, I would assume it can apply to general illness and ailment as well.
Conceptual Development: The Gnomish Lexicon of the 1910s had G. côma {“disease, pestilence, sickness” >>} “disease, illness” (GL/26), related to ᴱQ. qáme “sickness, nausea” and thus based on the early root ᴱ√QAMA (QL/76). This became ᴱN. côm in the Early Noldorin Grammar of the 1920s (PE13/123) and then {cóm >>} ᴱN. pau “sickness” in Early Noldorin Word-lists a bit later in this period, still cognate to ᴱQ. qāme (PE13/140, 152). The new form reflected changes in Noldorin’s phonetic developments: kw > p and ā > au instead of earlier (Gnomish) ā > ō and kwo > ko. These Gnomish sound changes were allocated to Ilkorin in the 1930s: see Ilk. côm “sickness”, also from The Etymologies of the 1930s (Ety/KWAM).